Sokka's space sword. The sharpness of the sword actually works against Sokka in the finale. When he tries to slow himself and Toph down by stabbing into the hull of an airship, it shears through the metal until they reach the bottom and keep falling.
To a lesser extent, Zuko's sabres. Zuko uses them to deflect stones without them suffering so much as a dent.
Also Mai's throwing daggers, being capable of nailing people to solid metal just by throwing them.
Waterbenders can use water to cut through metal, sort of like a water jet cutter. In the first season finale, Aang slices apart the metal superstructures of Fire Nation ships using this.
FireLord Ozai. That scar on his son Zuko's face and his emotional turmoil, and his daughter Azula's heartless, sadistic nature are both his doing.
Taken to new levels in the graphic novels detailing what happened to Ursa. Ursa lived in a small village as a theater performer and was betrothed to a fellow actor when it was decided she would be Ozai's wife. She was told she would have no contact with her former beloved or he would be killed. Secretly, Ursa was writing letters to him that she gave to a servant to deliver in secret, but the servant was taking them to Ozai. When she found out and confronted Ozai about it, he points out a part in one of her letters where she says Zuko is her former lover's son. He knows this is only wishful thinking on Ursa's part since he has her under constant watch, but he punishes her by treating Zuko as if he really was not Ozai's son.
Toph's parents are mostly just neglectful, sheltering and coddling their girl instead of respecting her earthbending skills, but sending bounty hunters after her when she runs off (and assuming she must have been kidnapped by the Avatar, or perhaps her father being afraid of the damage Toph herself could inflict after seeing how powerful she truly was) crosses over into stupidity.
Mai's parents—though not as bad as Zuko's or Toph's cases, it's revealed that she couldn't do much of anything except sit still and be quiet. If she made a comment at a dinner party, she got in trouble, if she fidgeted, she got in trouble. Heck, if she hugged her dad in public, she probably got in trouble. All because her parents just wanted to get higher and higher on the social ladder...and then they pretty much put her aside when her little brother Tom-Tom was born.
While not as bad as the others, Ty Lee's parents are said to have been rather neglectful. As one of seven identical septuplet girls she was constantly ignored, if not mistaken for one of her sisters. This treatment eventually led her to run away from home and join the circus.
All sorts: the danger of having your family die, the inescapable life of a refugee, inability to keep your loved ones safe....
...A foreign nation invading your homeland, an oppressive government controlling every facet of society, being rejected and betrayed by your loved ones, being forced to choose between your loved ones because they have taken up opposite sides of a conflict, and losing control of yourself and causing someone else to be hurt or killed. When Avatar plays for Emotional Torque, it plays for keeps.
Adults Are Useless: Zigzagged. It's mostly played straight but with a few major exceptions. If someone's an aversion, you can bet they're a member of the Order of the White Lotus.
The names for most cultures seem to be Chinese and Japanese mixed together, with a few other things thrown in (Zuko, and Iroh look like Japanese names, Zhao and Ozai are Chinese; they're all from the Fire Nation). This trope happens if you're Chinese or Japanese - names that sound normal to you mixed with names that really don't. There are also a few names that seem "normal" to Western audiences, like Lee, Mai, and Jet.
Notably, the few Vietnamese names in the show belong to Mai and the three guys from the Foggy Swamp Tribe.
Some of the names that are not as clear are Toph and Azula:
According to the characters written in certain episodes, Toph is an alternate spelling/pronunciation for either of the Chinese names Tuòfú or Tuofu. No word on whether that was the intention from the beginning or if they were just retroactively trying to find characters that fit. The Other Wiki says that Toph's name actually means 'supported Lotus' in Mandarin. Her mother's name is Poppy however.
Azula's name looks Japanese, except for the fact that it contains the letter 'l'. Her name was originally to be "Zula" (to mirror Zuko), but was changed to Azula to capture the Spanish word Azul (blue) and create a Meaningful Name for the girl slinging blue fire around, albeit one which is left-field in a world that is obviously South and East Asian-themed. Alternatively, in keeping to the Japanese language, you can use the r/l switch and get yet another Meaningful Name for "blue".
It would seem that Azula was named after her grandfather Azulon, who is named after Azulong, or The Azure Dragon of the East, a Chinese constellation symbol.
Double points for Zuko, whose name is not only both a Japanese and Chinese name, but is also a Meaningful Name in both languages. In Chinese, the name Zuko means "Conceited, proud of one's self", which seemed to fit his character rather nicely for a good portion of Season 1. In Japanese, the name 'Zuko' also means "sound of swishing swords", clearly in reference to both his swords and skilled swordsmanship.
Zuko is also fairly close to the Filipino word "Suko", which can mean "madness" or "angry" in one dialect, or "surrender" in another. While the second meaning doesn't fit him, the first meaning definitely does, especially in the first season.
You could make a case that, in Book 3, Zuko finally lets go (i.e. surrenders) all his anger.
In the episode "The Serpent's Pass" the travelers who are being escorted through the Serpent's Pass decide to name their child born just after they reach the other side Hope.
One of the partygoers in "The Beach" is named "Ruon-Jian". It's pronounced "Ron Jon".
Zuko's mom is named Ursa, which is Latin for bear.
An Aesop: Lots of them. Many, many episodes have morals to impart, such as "The Kyoshi Warriors" ("sexism is bad"), "The Fortuneteller" ("we make our own destiny"), and "The Deserter" ("impatience is bad"). Few of them are broken, lost, or otherwise faulty—or so Anvilicious as to feel like being on the receiving end of a sermon.
Azula is a terrible, terrible person, and while it's possible to feel sorry for her, usually she's such a magnificent bitch that you temporarily forget those feelings. Then she spends the whole series finale going steadily crazier and crazier, with her Freudian Excuse coming to the surface. Then she finally snaps after she loses to Katara and the last shot of her in the series is her sobbing uncontrollably, with Zuko and Katara themselves looking sad about it.
Fire Lord Sozin dies offscreen, but he gets a lot more sympathetic by then, realizing that he was irredeemable after he betrayed and murdered his best friend and wiping out the Air Nation, and has nothing to show for it except an empty, hollow life.
All Monks Know Kung-Fu: Every single one of the Air Nomads knew airbending due to their culture's high level of spirituality. Also, the Fire Sages are all master firebenders.
Justified in-universe, since bending is a highly spiritual discipline.
All There in the Manual: The main purpose of the Nickelodeon site is to provide supplementary information while not using up valuable air time.
The Avatar comics which appeared in various Nickelodeon publications throughout the run of the series also fill in some important details, particularly the ones set between Seasons 2 and 3, which reveal what happened to Ba Sing Se after Azula conquered it, and how Zuko and Mai got together.
Alternate Character Interpretation: With Zuko, In-Universe. Aang, Sokka, and Katara treat him like a violent sociopath when he arrives at the Western Air Temple, because from their perspective he is. Zuko has only ever shown up to either assault them or, in the case of the Crystal Catacombs, stab them in the back after making peaceful overtures. They've never witnessed the same inner doubts and character growth that the viewers have. Only Toph has no history with Zuko, which is why she's the one willing to reach out to him first.
Occasionally subverted throughout the series as we see some of the Fire Nation are just Punch Clock Villains and behave relatively normal when away from combat, and then fully subverted as the third season goes on and we meet the citizens of the Fire Nation, and even made fun of in one of the comics, where Sokka insists they spy on a Fire Nation village, and we see the people doing completely normal, innocent activities; Aang even lampshades the subversion at the end of "The Avatar and the Fire Lord": "Everyone is capable of great good and great evil." And then there's Hama, the southern Waterbender who subverts Always Lawful Good.
One can even initially perceive the element of Fire itself as this, as most depictions earlier in the show show it as destructive and violent. Later also subverted fully in "The Firebending Masters", where Aang and Zuko find out that at its core, Fire is energy, and life.
Always Save the Girl: Aang decides saving Katara is more important than finishing opening the chakras for the Avatar state, which was something that Iroh fully agreed with.
Ambiguous Situation: Pretty much everything surrounding Azulon's death. Was he really going to have Zuko killed to punish Ozai, or did Azula lie about that and Ozai just ran with it to get Ursa to help him kill Azulon? The only people who know for sure are known liars, and it's only ever discussed while they are trying to manipulate someone.
Ty Lee shows unusual interest in Azula, which goes almost to the point of obsession. On Ember Island, several boys took interest in Ty Lee on the beach, to which she was completely oblivious and preferred the company of Azula. She also had a brief interest she took in Sokka.
Then again, Ty Lee lives in complete and total fear of Azula so maybe her "interest" is just playing the part of a good employee to avoid getting on her bad side.
Additionally, Azula was originally conceived as a guy so perhaps Ty Lee was intended to be the girlfriend to the original male version of Azula, with some bits and pieces of that characterization remaining.
The Avatar Extras get into it too. At the end of "The Chase", when the Gaang plus Zuko all attack Azula, it says, "This is the first time we've seen all four elements attacking at once." A beat later, it adds, "...Plus Sokka."
Angrish: Sokka's reaction towards Toph waking him up in "Bitter Work". After considering the sleepless marathon he had to go through in the previous episode, his frustration is well justified here. The Avatar Extras note that the writers did have actual dialog for Sokka, but the voice actor improvised the grumbled angrish instead.
invokedAngst Dissonance: An in-universe example; while Sokka and Zuko are heading towards the Boiling Rock and bonding, Sokka says, "My first girlfriend turned into the moon." After a pause, the only thing Zuko can say is, "That's rough, buddy." What else can he say to something like that?
Angst Coma: In Season 2, Zuko undergoes a severe fever and enters a coma in which he has vivid dreams in which his uncle and sister appear as dragons and argue over his life choices. Iroh says that this is "not a natural illness" and the whole thing is apparently caused solely by Zuko's premature Heel-Face Turn.
"I must regain my honor." There are story sequences built around this phrase for Zuko, Aang, and Sokka ("The Boiling Rock")
"Destiny" is another major theme, seeing as the show is primarily influenced by Eastern Philosophy. It ends up applying to every single character, and together their stories send the message that while you always have a destiny, you are the only one who can choose to follow it.
While not exactly a major theme, the word "crazy" is used repeatedly: It's associated with Azula several times ("...crazy blue firebending", "Girls are crazy!", "She's crazy and she needs to go down.") and is also used to dismiss ideas and people as unimportant. Sokka uses it a lot during "The Fortuneteller"; during "The Beach", Toph says: "Guys, you're all gonna think I'm crazy, but it feels like there's a metal man coming." By the end, Azula becomes the sort of person who's a shorthand for "unimportant" and "easily dismissed."
The first season is bookended by Iroh telling Zuko "A man needs his rest."
Lampshaded by Katara crossing her fingers and repeating "community service". Gotta love it.
The Earth King's first line in his titular episode also makes good use of this trope.
Earth King: You invade my palace, lay waste to all my guards, break down my fancy door, and you expect me to trust you?! Extras Popup: And he loved that fancy door! And do I even need add "the malicious destruction of cabbages"?
Sozin's "Comet" is something of a misnomer. Comets are seen for days or possibly weeks as they take their sweet time to fly around the sun, are made of much more ice and earth than fire, and rarely intersect the atmosphere and live to tell their tale again. There is a class of objects that do—Earth-grazing fireballs—but "Sozin's Earthgrazer" isn't anywhere near as poetic, and doesn't carry that ancient "comets are harbingers of doom" mystique. In addition, the distinction wasn't always as clear in our world, either, so a culture with no advanced astronomy could easily just call any flying space object comets, and the word "comet" comes from Ancient Greek, the language of a culture in which people also thought the world consisted of a total of four (guess which) elements.
Word of God owns up to the eclipse's visibility all over the world being impossible.
It is plausible if Omashu and the Fire Nation Capitol just happened to both be in the swath covered in total eclipse. Intercontinental total eclipse zones are possible in real life◊.
Real total solar eclipses are not at all as rare as the show makes them out to be - they must happen at least once every 5 years, but can go centuries before recurring at the same location. Before the team could have used the planetarium to show when an eclipse would happen over the Fire Nation, they would therefore have had to program it to show the sky at that location.
In the days just before the eclipse, the moon is shown full or near-full several times. A solar eclipse can only happen during a new moon, when the moon is on the same side of the Earth as the sun.
A similar issue appears in "The Siege of the North." Despite being located at the north pole, the sun rises and sets as it would below the arctic line. As mentioned in "The Boy in the Iceberg" by Sokka, midnight sun does in fact exist in the world of the Avatar. As such, for day and night to cycle during a normal twenty four hour period in the north pole is not plausible around the solstices. That said, it was not said how long after the winder solstice it was until Aang and his friends reached the North Pole.
It can be argued that, seeing as the Northern Water Tribe capital is on the coast, they could still be fairly far from the "true" North Pole.
Fire does not have a concussive effect unless it is part of an explosion, and even then it's the rapidly expanding gases (fire is a form of plasma) that cause the person or object to be pushed back.
Mai's daggers should not cause people to go flying back into walls unless she's somehow Gambit in disguise. Also note that at the speed she throws them, they should just cut right through the clothes instead of dragging people with them.
Though there are some uncommon aversions as well: Azula's fire is extra hot and therefore blue, and Aang makes a point to cut the surface tension of the water he falls into, also averting Soft Water.
Art Evolution: In a short series like this, and an animated one to boot, one normally doesn't find art evolution. However, it does occur; for instance, compare the size and shape of Katara's eyes (they shift from a noticeably Tibetan, or maybe Inuit, appearance to the more standard large anime eyes) in the first few episodes to the last few episodes, Sokka's entire facial structure and even Aang's face changes somewhat.
Also, in a more subtle example, as time goes on, Zuko's scar seems to become a much less dominant feature of his face, partially because he loses his Bald of Evil and lets his hair grow long enough that it covers a lot of the scar.
Ascended Extra: Jet, and to a lesser extent Longshot and Smellerbee, in Season 2. Suki may count as well.
In a more literal case, the couple Zuko decides not to rob in "Zuko Alone" when he sees the woman is pregnant are the same couple that the Gaang helps out in "The Serpent's Pass" and then end up sitting next to Zuko on a train going into Ba Sing Se.
Ascended Fridge Horror: In "The Puppetmaster", Katara learns she can use her waterbending on the blood in people's bodies to control them like puppets.
Asleep for Days: Aang is knocked unconscious by Azula's lightning bolt so long that his hair had enough time to grow back.
Asshole Victim: All signs point to Fire Lord Azulon not being a very nice person. Word of God and background material show him as ruling the Fire Nation with an iron fist and carrying out the war against the other nations with brutality only matched by Ozai. Like the genocide carried out against the Southern Water Tribe's waterbenders and the raids carried out against said tribes (these raids eventually resulted in the death of Katara's mother). He was willing to have his innocent grandson killed just to prove a point to Ozai. Other than possibly Iroh, it's unlikely anyone really missed him when Ozai and Ursa killed him.
Assurance Backfire: In the episode "The Cave of Two Lovers", a cave-in splits up the party in an underground tunnel, and Sokka ends up alone with a group of singing nomads. As Sokka tries to dig his way through the debris, the nomad leader comments, "Yeah, it's no use. We're separated. But at least you have us!" Sokka's response is to scream in despair and start digging more frantically.
Zuko after his Heel-Face Turn, and before as well. Before, he's trying to atone for the event that led to his dishonour and banishment. After, he's trying to atone for betraying his uncle Iroh in his attempt to get back into his father's good books.
Jet in his second appearance.
Iroh seems to be atoning throughout the show for things he did before it even started.
At the Crossroads: The Season 2 finale is "The Crossroads Of Destiny" where Zuko and Aang make their tough choices.
Attack Its Weak Point: Sokka defeats Combustion Man by throwing his boomerang into the center of Combustion Man's eye tattoo. This causes it to malfunction and explode when he tries to fire his lasers again.
Also, the Gaang use this tactic against the drill attacking Ba Sing Se, in the episode "The Drill".
King Bumi, who managed to take over an entire city by himself, using his face.
The entire Fire Nation royal family.
Any Grandmaster of the White Lotus. Even Piandao, the only known non-bender member of the order, is incredibly powerful with a simple swordnote Though you could describe a few of the members as 'grand masters' of their martial arts, only Iroh was a 'Grand Lotus' in the Order.
Aura Vision: Ty Lee has the ability to see other people's auras. Hers is pink and Mai's is gray. No word on the other characters, though.
Several characters; Toph, Iroh, and Azula being the biggest ones.
Honorable mentions include Zuko, who is mostly outclassed by his enemies and family, but otherwise can hold his own even without firebending, and Aang, a Physical God 12 year old Martial Pacifist who ended a century long war almost by himself.
Practically required to be in the Order of the White Lotus. A bunch of very old men retake the impenetrable Ba Sing Se (which was taken through espionage and trickery) in a full frontal assault on the day when the fire benders are strongest.
Especially Bumi. Bumi does not escape from the army occupying his city and holding him captive; the army escapes from Bumi.
In addition to retaking his own city (and escaping captivity with only his face) Bumi used this to his advantage when he first met Aang after the 100 year time skip. Knowing Aang would try to outsmart him, Bumi put himself in a position where Aang can try to Loophole Abuse his way into fighting a frail old man instead of his two large and muscular guards. Aang learned the hard way why Bumi was king.
Aang's mentor, Gyatso, counts as this. When his corpse is discovered by Aang, it is found in a tent that is completely littered with Fire Nation armor and skeletons. Despite the Air Nomads' "all life is sacred" vow, it's clear that Gyatso was extremely powerful and took down dozens of Fire Nation benders and soldiers who's firebending was amplified from the effects of Sozin's Comet, and the only way they managed to kill him was by sending wave upon wave against him until sheer number finally broke him down. Badass indeed.
Badass Minds Think Alike: Aang and the Blue Spirit worked extremely well, considering it was the first time they fought together, they had done no planning, and that the Blue Spirit doesn't talk.
Which makes it all the more ironic when the Blue Spirit turns out to be Zuko.
Badass Normal: Sokka, Hakoda, Suki, Ty Lee, Mai, Jet and his freedom fighters, Piandao.
The "Sokka's Master" episode may count as a lampshade When Sokka is dissatisfied with his being normal, the group suggests that he Take a Level in Badass.
Bad Bad Acting: "The Ember Island Players". Also, when the Gaang tries to get Katara arrested for earthbending in "Imprisoned".
Badbutt: Toph, especially while training Aang, and Jet.
Bad Omen Anecdote: In the episode "The Cave of Two Lovers", the hippie nomad who tells the Gaang about the secret cave, sings to them a song about it. The song ends with "...aaaand diiiiiiiiiiie!".
Bad Powers, Bad People: The overall public opinion of firebenders. The truth is a bit more complicated than that, though. See the trope page for more details.
Sokka lampshading this: "Mind if I watch you two jerks do your jerkbending?"
Bare Your Midriff: Quite a few ladies in the show. All the main female characters of the show have done it at least once, and for Ty Lee, it's part of her normal outfit. It also seems to be pretty common in the Fire Nation and seems to even be a normal part of the female soldiers' uniforms.
Aang is very attached to Appa, and Katara, and putting either in danger is a very good way to have him go Avatar-state crazy on your ass.
"YOU MUZZLED APPA?!"
Sokka loses it when Azula taunts him with descriptions of his captured girlfriend. For context, he knew she was taunting everyone to get them to run out the clock and he still lost control. Granted, Azula's just that good at manipulation.
He ends up fighting with Hahn over Yue, who the warrior clearly doesn't appreciate.
When Aang accidentally burns Katara, Sokka tackles him to the ground. Nobody hurts his little sister.
Anything that reminds Katara of what happened to her mother is enough to send her flying out of control, to the point where she bloodbends a man whom she thought was the murderer. She eventually learns to rein in this part of herself.
Harming Aang is her second berserk button.
Iroh rarely gets truly upset throughout the entire series. However, he has a deep connection with the spirits, which is why we see him none too pleased when he realizes Zhao plans to defeat the Northern Water Tribe by killing the moon spirit.
Iroh: [to Zhao] Whatever you do to that spirit, I'll unleash on you tenfold! Let it go NOW!
Jet and absolutely anything having to do with the Fire Nation. His inability to let go of his intense Fantastic Racism is what inadvertently leads to his death.
Berserker Tears: When Sokka discovers Azula captured Suki and the rest of the Kyoshi Warriors (including referring to Suki as "[her] favorite prisoner") during the Day of Black Sun, Sokka gets these and flips his shit.
Bits of Me Keep Passing Out: Sokka during the fight with Ty Lee; she was paralyzing his limbs one by one, yet he still tried to fight. Another similar incident happened when he was paralyzed by June's beast, and just when he started to gain some control over his hand, a pile of blocks fell on top of him.
Bizarre Alien Senses: Toph's ability to "see" using earthbending. It is very unusual when you think about it since it requires forming "images" through contact with the ground. The closest equivalent in real life would be how snakes and elephants can detect low-frequency sounds through vibrations in the ground as well.
Black and White Morality: Played straight at the extremes: Aang and company are good, Ozai and Azula are bad. Subverted with prejudice practically everywhere else, as a wide variety of sympathetic Fire Nation characters are introduced, while both the Earth Kingdom and the Water Tribe each get at least one villainous figure. One of Aang's most important lessons is that good and evil can come from anywhere.
Black and Grey Morality: Concerning the war between the Fire Nation and the Earth Kingdom itself, it becomes this. The Fire Nation as a whole is basically a fascist empire trying to take over the world, and among their methods of dealing with the enemy include imprisoning civilian earth benders and emotionally breaking them, and dressing up captured soldier in their own armour and sending them to the front lines without weapons to be slaughtered by their own forces. The Earth Kingdom however are mostly trying to fight them off, but their soldiers can sometimes be incredibly corrupt, their leaders are willing to do unspeakably horrible things in an attempt to find some advantage.
Aang: You could call it luck... or you could call it lying. I made the whole thing up. Katara:[amused]: You did not. That is so wrong.
Also, when Toph's father tells two bounty hunters that Aang kidnapped his daughter when really, he knows that Toph ran away in defiance.
Bluff the Eavesdropper: Azula has Mai and Ty Lee purposely reveal that they're Fire Nation in disguise... to the pair of Dai Li agents they knew were eavesdropping, overhead. She knows they'll ferry the news back to Long Feng, and that he won't be able to resist the opportunity to use it against her. So she isn't surprised when those same agents bring her to his cell, where he coerces her into helping him under threat of exposing her to the Earth King. Azula "reluctantly" agrees. It doesn't end well for Long Feng.
Subverted in that we eventually find out how the fight turned out.
A very subtle example may occur in "Lake Laogai". With Jet mortally wounded, his comrades Longshot and Smellerbee tell Aang's group to go on ahead. This is followed by a shot of Longshot nocking an arrow in anticipation of incoming enemies. The trio are never seen again, not even when nearly every living good guy teams up near the end of Book 3.
This one was also subverted as of the sequel comic "The Promise", where Smellerbee and Longshot make an appearance.
Bond Villain Stupidity: The Fire Nation in general. For a nation dedicated to wiping out everyone with Elemental Powers besides themselves, they seem remarkably intent on leaving the Benders from other nations sitting around in prisons rather than just executing them - and confirmed traitors like Iroh, as well. Imprisonment would be a more viable choice since execution of the benders may turn more people against them including the very citizens of the Fire Nation. From the public's point of view, killing an enemy in the battlefield is different from executing an already defeated and imprisoned foe. This is a kid's show that tends to take a Never Say "Die" attitude with respect to named characters. Execution is a bit... adult.
It still can be justified in-universe: Keeping in mind than the War lasted one hundred years, killing each new generation of benders would establish the Fire Nation as unbelievably evil from the point of view of non-benders, that they can't subdue due to their sheer number (The Air Nomads being the exception, since they were a sufficiently small population to Kill 'em All at once). Simply neutralizing them work better from a P.R. stand-point.
Book Ends: A pillar of light shooting into the sky in the first and last episode.
For the war, Sozin's Comet.
Boring, but Practical: Very commonly used throughout the show. Weapons and armor worn by the characters tend to be much closer to actual equipment used in history with very few unnecessary frills.
The Fire Nation Navy used to have shoulder spikes on their uniforms, but eliminated them soon during the war due the spikes constantly getting stuck into things.
The best part? This was the creator's answer to Nickelodeon's request for them to give Aang armor so they could make it into a toy.
In the beginning, Zuko rages at Iroh over the latter's training focusing on basic firebending. Later, we see that Zuko's mastery of the most basic part of firebending, control of one's breathing, gives him a resilience to cold that other firebenders can't match.
Boobs of Steel: Ty Lee has a rather sizable chest, the largest in Azula's group, and is easily the physically strongest and most agile character in the series who isn't using bending, male or female (her name even means "extreme strength").
Entirely averted with regards to bending. Neither Katara, Toph nor Azula are especially well endowed for their ages, but all are among the best in the world in their respective elements. Indeed, Toph may well be one of the greatest Earthbenders in history and she's as flat chested as one would expect a 12 year old girl to be.
Lampshading this aversion might explain the fanart Running Gag of making older versions of Toph particularly well-endowed... or possibly just so the artists have an excuse to joke about the Earthbender carrying around a couple of huge boulders.
Break the Cutie: Pretty much every character ever. Katara, Sokka, Aang, Zuko, and Jet all get flashback scenes showing them as cute, chubby-cheeked little kids that have horrible things happen to them. The rest of the Freedom Fighters, Ty Lee, and Mai are implied to have gone through this - all of the Freedom Fighters are orphans who have been hurt by the war, Mai withdrew into herself as a result of her oppressive childhood, and Ty Lee snapped and ran away to the circus because she was treated like she had no personal identity. Even Azula could arguably fall under this - her deep desire for her parent's affection, likely based on her mother's unspoken preference for Zuko, is part of what makes her the monster she is during the show.
Don't forget Appa and Hama. Appa had an entire episode showing him being treated as a universal punching bag. It was horribly depressing.
At some point, the group oohs over a cute portrait of "baby Zuko." But the portrait turns out to be of his father, Ozai. It is hinted that Ozai's father Azulon was a nasty character.
Break the Haughty: Zuko over the course of most of the series, which leads to serious character development on his part; while Azula (who starts out even haughtier) gets it all crammed into a few episodes and doesn't take it as well.
In the Book 1 episode "The Deserter", Sokka tries some Fire Flakes, which are pretty hot, but it's made much funnier when we meet Mai a season later in "Return To Omashu" and she snacks down on them like they're nothing.
Sokka developed a taste for them by the time "Ember Island Players" rolled arond.
Zuko mentioning the tsungi horn in Book 1 seems like an offhanded comment at first, if you don't remember that it actually appeared in "The Waterbending Scroll"; later we see Aang playing one in "The Headband". Iroh himself plays one in the last episode (presumably it's the one they bought).
You can hear the "tsungi horn" - actually a duduk in Zuko's Leitmotif.
From that same episode, Iroh can be seen admiring a ruby-encrusted ape statue which then appears in his possession in "The Blue Spirit".
In the beginning of Book 3, Aang makes a picture of Fire Lord Ozai at school. In "Daydreams and Nightmares" it's put on a tree so Aang can train.
"Boomerang! You do always come back!"
To a lesser extent, Sokka's original nickname for Combustion Man kind of fits. In the first half of the episode "The Runaway", there's an apparent throwaway gag where Sokka giddily tells the group he came up with the nickname "Sparky Sparky Boom Man"; he mostly just gets ignored as Toph and Katara continue arguing. Then toward the end of the episode when Combustion Man makes a surprise attack, Aang, with a completely straight face, delivers the line "It's Sparky Sparky Boom Man!"
In "Daydreams and Nightmares", Aang has a reoccurring "Not Wearing Pants" Dream about fighting the Fire Lord only to discover he's not wearing pants, until he eventually gets over it, saying: "No, Fire Lord, it is you who is not wearing pants!" "My royal parts are showing!" In the finale, Aang and Ozai are both shirtless for most of the fight, so they are both only and specifically wearing pants.
In book one Sokka is trapped in the spirit world for a whole day and afterwards claims he really needs the bathroom, In book 3 Katara asks if they have bathrooms in the spirit world to which Sokka replies "As a matter of fact, they do not"
Iroh also qualifies, though he hides his pain behind an upbeat, silly façade most of the time.
Broken Glass Penalty: In the Iroh segment of "Tales of Ba Sing Se", a few kids are playing with a ball, and a window is broken. Iroh appears and the following dialogue occurs:
Iroh: It is usually best to admit mistakes when they occur, and to seek to restore honor. A large man appears inside the house Large man: When I'm through with you kids, the window won't be the only thing that's broken! Iroh: But not this time! Run!
Brotherhood of Funny Hats: In the comic book story "No Benders Allowed" (printed in The Lost Adventures), Sokka starts a club like this along with the Gaang's non-bender allies. They eventually let the benders into the club, provided they "pledge allegiance to my bendless brethren, and admit that no bending can equal the might of the noble boomerang".
Bubble Pipe: Sokka when he's doing his Sherlock Holmes impression.
Bullet Time: Ample examples throughout the show, with Aang having the most.
Prominent in Toph's first episode; time freezes many times, showing how she feels the vibrations in the ground to "see" what's going on. In that same episode, a rock flies by her face in slo-mo.
Also done during Pakku and Katara's fight in "The Waterbending Master."
Bullying a Dragon: When Aang encounters Zhao in The Deserter, the whole battle is Aang antagonizing Zhao so the latter will try to blast the former with fire. Zhao didn't realize he had been blasting his own boats until Aang pointed it out.
The Bus Came Back: This show is notable for bringing characters who appeared at first to be one-shot extras back for reappearances (often spoiling the surprise with a Spoiler Recap at the beginning of the episodes in which they reappear).
Most notably, in "Day of Black Sun", characters from "Jet", "The Northern Air Temple", "The Swamp", and "The Blind Bandit" show up… as well as Hakoda, Bato, and the warriors of the Southern Water Tribe. A number of characters in the finale, too (see Back for the Finale, above).
Jet, Longshot, and Smellerbee in Season 2.
Also, Suki at various points: "The Serpent's Pass", "Appa's Lost Days", then becoming a Guest Star Party Member from "The Boiling Rock" until the end of the show.
On the villain side, the pirates from "The Waterbending Scroll" showed up again in "The Waterbending Master".
June the Bounty Hunter, first appearing in Season 1's "Bato of the Water Tribe", and then reappearing in Season 3's "Sozin Comet, Part 1: The Phoenix King".
Zuko: Why'd you do it?! Azula: You're going to have to be a little more specific...
A more humorous version of this happens when Sokka and Zuko travel to the Boiling Rock and run into Suki:
Sokka: Oh, good. You guys have met. Suki: Actually, we met a long time ago. Zuko: We did? Suki: Yeah. You kind of burned down my village. Zuko: Oh. Sorry about that. (beat) Nice to see you again?
This example is a bit odd in that it seems it was a bit of a Tuesday for Zuko and Sokka. While he was obviously present for the whole episode, Sokka apparently remembers Suki but not Zuko setting fire to her village. Maybe justified because Suki gave Sokka his Sacred First Kiss?
Sokka, on several occasions, particularly during the first half of the series. Sokka actually displays considerable growth over the course of the show, something few Butt Monkeys in other shows ever get to do.
In line with his Break the Haughty and Broken Bird status, Zuko is quite a butt monkey in his own right. What with his upbringing from the loss of his mother, the fact his grandfather wanted to kill him as an adolescent (and the way his sister gleefully points it out to him), his relationship with his father, the constant humiliation and one-upping he got from Azula, and the lumps he takes throughout his quest to capture Aang, destiny really wanted to make sure Zuko was good and tempered when the time came to face it.
The Cabbage Merchant.
By Wall That Is Holey: the Earth Kingdom's trademark disc projectiles with square holes in them (based on Chinese coins) turn out to be rather impractical, particularly in "The Avatar State".
Azula: Isn't it obvious? I'm about to celebrate becoming an only child!
Call a Smeerp a "Rabbit": "Penguins". Played for laughs and subverted with the Earth King's pet bear; the crew goes on to name various animals that could be combined, but are left perplexed as it just says 'bear'. This trope is pulled throughout the entire universe of Avatar, which lends more humor to the situation.
Call to Agriculture: Subverted. The man who killed Katara's mother has a garden and it is implied that he spends quite some time on it, but he is still the cold and heartless man he was when he was an Admiral. It's just hard to tell because Katara is being really scary, and he's already a coward anyway.
During Season 2 and at the end of the Grand Finale, Uncle Iroh is "Called To Tea Service".
Calling Your Bathroom Breaks: The children that Sokka is trying to train in the first episode, just to illustrate how young they are and how hopeless Sokka's task is.
Though he's only partially correct himself, Jet goes through this when trying to warn people that Zuko and Iroh are firebenders that are trying to infiltrate the city. (He's right about the firebenders part.) After spending several days trying to get proof, he loses all credibility when he decides to suddenly attack them in front of a bunch of customers to get them to firebend in defense, which leads to his capture and Brainwashing by the Dai Li.
In Jet's first appearance, Sokka also went through this while trying to convince Aang and Katara that Jet planned on wiping out an entire village just to take out some Fire Nation soldiers stationed there.
Cat Smile: Jin, who owes a lot of her popularity to this.
Charles Atlas Superpower: Anyone with an extended lifespan is doing so through nonbending means; for instance, the Guru and Avatar Kyoshi both lived over 150 years, even though the Guru has no bending ability.
Charge Into Combat Cut: This trick is used a few times in the finale to cut between Aang's battle with Fire Lord Ozai and Zuko's battle with Azula.
Character Exaggeration: "The Ember Island Players" is an in-series example where just about everyone is absorbed by some single trait, often a comparatively minor one that they may not even have anymore.
Subverted with Haru in the episode "Imprisoned". He saves an old man with his illegal earthbending. Old man turns him in...
Chekhov's Gag: In the earliest episodes, Katara had a tendency to mess up her waterbending, and Sokka would get drenched. In "The Waterbending Master" Katara redirects a stream of water during a fight. Sokka gets blasted away by it, even though everyone else around him is just fine.
Early in Episode 5, Aang mentions his friend Bumi, a kid with mismatched eyes and bad teeth. At the end of the episode, the king of Omashu, who has mismatched eyes and bad teeth, gives Aang one final test. It's to guess his name: Bumi.
The warden's word that he would sooner jump into the boiling lake than tarnish his prisoners' escape record (then, zero). One episode later he is willing to have the gondola cable cut to prevent our heroes from escaping, even when it means his own demise.
Zuko's twin swords (seen on his wall early in the series), schematics for a drilling machine, Katara's necklace twice (one used by Zuko to track the Gaang and once when Master Pakku realizes it belonged to Kanna, Katara's amulet of water from the North Pole oasis, the Lotus tile, Iroh's sandal, the scroll with the Lion Turtles at the Library and the bison whistle in particular since Sokka initially chided Aang for wasting money on it. One season later, it's still important.
The bison whistle is more of a Chekhov's Boomerang: It apparently serves its purpose at the end of the episode it first appears in, only to show up again halfway through the next season.
The war balloon is a special one that deserves mention because it takes two seasons to resolve.
After Katara steals the waterbending scroll, she attempts to learn the water whip. Look at the diagram. It's the same waterbending move in the intro every week.
The same waterbending scroll is what allows Katara access to the Library in a later episode.
From the start of "The Waterbending Scroll," they set up a Chekhov's Gun that really plays out only if you're really paying attention closely. Iroh mentions he lost his Lotus tile and remarks that a certain strategy he uses often revolves around it. At the time, you assume he's being the bumbling old man he's been since the show started. Fast-forward to Season 2, where he sits down to play Pai Sho with an old man and uses the White Lotus Gambit, which the old man recognizes and sets up his board accordingly. The result? The old man and Iroh create the sigil for the Order of the White Lotus, who serve the purpose of sneaking Iroh and Zuko into Ba Sing Se. The lotus tile makes another reappearance, as Pian-Dao leaves one for Sokka after Sokka's apprenticeship is completed, and in the end, the Order of the White Lotus makes a reappearance as the army set to reconquer Ba Sing Se in Season 3. This was a use worthy of Rowling!
A minor one in "The Avatar State," Zuko and Iroh cut off their top-knots with a knife Zuko has on him. Guess what pops up six episodes later in "Zuko Alone?"
In "The Library", Aang finds a picture of a human with a lion-turtle and comments on the fact that they supposedly have mystical powers. In the third-to-last episode, just before going off to fight Ozai, guess who he encounters?
Guru Pathik makes a short appearance in 'Appa's Lost Days' well before meeting Aang. Hakoda also appears in the same episode, but only for a few seconds.
Also, Azula is seen in the Storm flashback of how Zuko got his scar.
The benders seen in the opening credits are not introduced for some time, if at all:
the waterbender is Pakku,
the earthbender was the original design for Toph before the creators decided to make her a girl instead (which got reused for Avatar Roku's earthbending teacher). It is also speculated that the Earthbender is the Boulder.
the firebender is Azula
Roku also appears in the opening credits despite not being named until three episodes in, and he didn't even get any lines until four episodes after that.
A vision of Toph in "The Swamp".
Imagery of the Lion Turtle has constantly reappeared throughout the entire series, but never in a way that would make its true purpose apparent.
The married couple that Zuko almost attacks in Zuko Alone reappear as refuges in the Serpent's Pass.
Made even more brilliant when you watch The Storm. Iroh redirects lightning during the titular storm. In season one. Mike and Bryan, I worship you.
Zuko using firebending to hold off hypothermia in "The Siege of the North" comes in handy when he gets locked in the cooler in "The Boiling Rock". (Which is arguably less fatal than freezing water, but still impressive.)
Katara's use of bloodbending in "The Southern Raiders".
Zuko wins the duel with Zhao in the second episode by spinning on the ground while launching fire from his feet, knocking Zhao off balance. He ends his duel with Azula in the finale with the same move, though with much more power, thanks to Sozin's Comet.
Chirping Crickets: Sometimes with a cough, but mostly with animal sounds - a duck in "The Fortuneteller", a defrosting frog in "The Blue Spirit" and a badger-toad in "The Western Air Temple", among others.
The Chosen One: Aang, although the reincarnation system makes it a bit muddled whether he's chosen or just following all his lives.
Katara was destined to be Aang's waterbending master. She always believed the Avatar would return and she released him from the iceberg. Gran Gran told her that their destinies "are now intertwined."
Toph was first seen as a vision and was therefore destined to be Aang's earthbending master.
Zuko (who, in keeping with the pattern of this series' use of this trope, is destined to be Aang's firebending master) just happened to be passing through the South Pole when Aang was discovered in the iceberg. He also found out that Avatar Roku was his great-grandfather and that his destiny was intertwined with that of the the Avatar. Aang even admits this outright.
Subverted with Iroh, who seems pretty out of it through much of season 1, but it turns out to be a case of Obfuscating Stupidity.
King Bumi is legitimately insane, but no less badass for it.
Played very, very straight with Ty Lee, a goofy Circus Brat who perpetually has her head in the clouds.
Ty Lee: Hey, look at that dust cloud. It's so...poofy. Poof!
Zigzagged with Sokka. Usually he's the Only Sane Man, constantly annoyed by everyone else's weirdness, but he definitely has his moments of insanity, such as when he notices the Team Pet has gone missing and arbitrarily declares him to have been eaten despite there being no evidence to that effect, even going so far as to climb inside a sky bison's mouth to find him.
Quite a few one-shot characters, most notably the travelling minstrels from the episode "The Cave of Two Lovers" who manage to annoy Sokka so much that he ends the episode with a red mark on his forehead from excessive Face Palming.
The clothing styles of Four Nations: Denizens of the Fire Nation wear red, gold, and black; The Water Tribes wear blue and white; the Air Nomads wear yellow and orange, and members of the Earth Kingdom wear greens and browns. The Air Nomads' colour is actually known as kavi, a variation of yellow worn by priests and monks. The writers did their research.
Also, Azula's blue fire, which indicates her cold, cruel detachment (It also shows her as a firebending prodigy since blue fire burns hotter) vs. Zuko's red (to indicate his heated emotional nature). Interestingly, the color combination was reversed for Aang and Ozai when they were energybending in the Finale.
There are exceptions to this rule, though many (but not all) are simply citizens of one nation disguising themselves as another.
Notable is Kyoshi Island. Despite being Earth Kingdom, many wear blue, almost like Water Tribe.
The characters' eye color seem to always matches up with their home nation. Justified with heredity and stuff.
In Book 2, Zuko and Iroh relocate to Ba Sing Se. In order to blend in they wear the customary clothing of the Earth Kingdom.
At the start of Book 3 the Gaang captures a Fire Nation ship and pose as Fire Nation. In the second episode, the Gaang steals Fire Nation clothing in order to blend in.
In the final episode the Gaang is dressed in Earth Kingdom clothing. This symbolizes the integration of the four nations.
Jet and his gang wear clothing not matching any of the four nations. (However, this could actually still work with this trope by going along with the fact that while they live in the Earth Kingdom, they are really more of a law unto their own, signified by their patchwork, lost boys-style clothes.)
Though still a shade of red, Ty Lee's pink wardrobe nonetheless stands out from the fiery colors of her Fire Nation comrades. And at the end of the series she's joined the Kyoshi Warriors and is dressed in their customary green and brown uniforms.
The Foggy Swamp Tribe is a tribe of water benders garbed in green and brown.
The color of benders' eyes correspond to the color of their element (green for earthbenders, blue for waterbenders, gold for firebenders, grey for airbenders) and to some degree, everyone's eyes tend to correspond to their nationality as well (which makes one wonder why no one suspected the blue/green/grey-eyed bunch of youngsters tramping around the Fire Nation of being the exact bunch of foreign youngsters they were expecting to try to overthrow the Fire Lord. In fact, Aang is identified as a foreigner on the first day, and they all begin to tell people they are "from the colonies.").
It could also be noted that, within the Fire Nation, eye colour does differ from person to person, often appearing to be coded by the characters alliance. For example, Zuko's eyes are light amber, signifying that, while he starts on the bad side, he is really good, Zhao's are dark orange, showing that he is ultimately corrupted, Azulas are so dark they are almost brown, and Iroh's are a nice pumpkiny yellow, showing his true alligence to the light.
There's also the Order of the White Lotus, who wear white clothing, signifying their willingness to mix philosophies and not be divided by nationality, since white is a mixture of all colors.
Color-Coded Patrician: Princess Yue wears purple, when everyone else in the water tribes wear blue, including her father.
Combat Hand Fan: Popularized by Avatar Kyoshi, it's the iconic weapon of the Kyoshi Warriors. In addition to being a handy melee weapon, it's very effective at boosting Airbending attacks.
Comet of Doom: Sozin's Comet (even if it's not REALLY a comet), although the prediction of doom is hardly mystical; its arrival increases the Firebenders' power by a hundredfold. Fire Lord Sozin, for whom the comet was named, used it to launch a first strike against the Air Nomads, which completely eradicated them. His grandson, Fire Lord Ozai, attempted something similar in the finale.
Commuting on a Bus: In the third season, this happened to Uncle Iroh... for the first few episodes, he didn't even get any speaking parts! (Word of God is that this had already been planned that way and was not because of the sudden death of his prior voice actor, Mako.) Then he had exactly four speaking appearances in the entire rest of the season.
Companion Cube: Sokka's Boomerang and later, Space Earth Sword. Complete with "death".
Confirmation Bias: An in-universe example in the series finale. Aang contacted the spirits of the previous four Avatars in the hopes that they would tell him that his waffling on whether or not to kill Ozai was justified. Each one of them failed to give him the answer he wanted, so he moved on to the next Avatar hoping they would tell him what he wanted to hear, in Kyoshi's case even saying "I knew I shouldn't have asked Kyoshi".
Conflict Ball: Passed around a few times, usually involving Katara.
Many, many examples. The umbrella from "The Fortuneteller" is found in Appa's luggage by sandbenders in "Appa's Lost Days". The eye-patch wearing Fire Nation commander from "Jet" shows up again in "The Cave of Two Lovers". The tsungi horn and ruby encrusted monkey statue Iroh buys in "The Waterbending Scroll" appear several times. War Minister Qin turns up at least once a season, usually prior to or during the unveiling of a new Fire Nation war machine. Sokka's attempt to reveal himself to Suki with a kiss in "The Boiling Rock" is a callback to when she did the same to him in "The Serpent's Pass". And so on.
Katara mentions in "Bitter Work" that the reason Aang is having so much trouble learning Earthbending is because Earth is the natural opposite of his element (Air). Later, in "The Avatar and The Fire Lord", Roku mentions conversationally that out of all the four elements, Water (the natural opposite to his element, Fire) was the one which was hardest for him to master.
The married couple first appears in Zuko Alone before appearing in the Serpent's Pass.
Iroh's journey to the spirit world was first referenced in the Winter Solstice Part 1 when he sees Spirit Aang. It is not actually mentioned until the Siege of the North.
The Badger Moles make their first appearance in the Cave of Two Lovers. Toph later mentions how she learned earthbending from the Badger Moles.
Additionally, the Badger Moles are seen briefly in season 2, episode 6 when Aang first decides that Toph should be his earth bending master. The moles are fixing the arena after the previous battles.
Characters often seen for brief moments make larger appearances in later episodes.
While Zuko and Iroh are hiding in Ba Sing Se, Zuko takes on the name Lee. While this is just a random name at that point, a season later we learn from Piandao that "There's a million Lees", so Zuko was essentially going by his world's version of "John Smith".
In The Boiling Rock, Zuko spends some time locked in a chilled prison cell, designed specifically to punish Firebenders, as part of a Batman Gambit by him and Sokka. He used the same chi technique that he learned at the end of the first season to stay warm, as demonstrated by his exhaled puff of flame when he is retrieved.
A particularly egregious example can be found in "The Awakening": Aang is standing among streams of lava, some of which flows literally between his feet. He sticks his wooden glider to the ground and leaves, and the glider immediately catches a fire and burns, but somehow this terrible heat doesn't bother Aang (or the other members of his gang) at all. So convection clearly exists, but it doesn't appear to affect humans.
Handwaved: According to the creators, Aang can use airbending to insulate himself from convection.
Conveniently Interrupted Document: Zhao burned away references to The Day Of Black Sun, and indeed, any references to the Fire Nation at all in Wan Shi Ton's library (which served as the last straw for the spirit, and caused him to become hostile to humans).
Cool Airship: In what doubles as one of the biggest Oh, Crap moments of the series, in season 3 the Fire Nation revealed that it DID steal the hot air balloon technology. They have perfected War Zeppelin technology in the meantime, building up a terrifying armada of them. They aren't just heavily armed with firebenders, cannons, and bombs; they also overwhelm and annihilate the heroes' escape route and give the villains—notably Azula—go-anywhere transportation. And they are undeniably cool.
Their biggest flaw, though, is that they are rather slow. Appa flew away from a zeppelin attack twice; in the Day of Black Sun, Azula remarks that they are too fast, even though Appa was weighed down with armor and extra passengers at the time.
Cool Old Guy: Numerous. Most notably, Uncle Iroh and Aang's mentor, Gyatso.
Cool Tank: The Fire Nation makes wide use of Steam Punk tanks with Firebenders as their weapons, but the coolest are the Earth Kingdom's tanks, which are incredibly tough worm-like contraptions powered entirely by Earthbending. As such, they can scale cliffs and staircases, attack conventionally with Earthbent rocks, or Earthbend their hulls to crush enemy tanks.
Could Have Been Messy: Many, many cases, on both sides. Aang will not kill people. He will, however, dump an avalanche of snow on top of masses of enemy soldiers while they're walking across a sheer mountain pathway seemingly very high up, sending them hurtling off. Just so long as the aftermath is unseen we can safely assume no one got hurt. Right?
The one time he does kill something (and when it was totally unnecessary) it's body is seen falling to the ground in the distance, darkened by the rising sun. The fleeing buzzard wasp's body was apparently split in half by his airbending, but we can't really see the details.
In the Siege of the North Part 1, when talking about getting the Ocean and Moon Spirits' support, Aang says that, "Maybe they'll unleash a crazy amazing spirit attack on the Fire Nation." Both Yue and Katara give him weird looks. Guess what happensnext episode?
Just before the series finale, Aang suggests, as an alternative to killing, that he simply encase Ozai's limbs in glue so he can't bend anymore. Alas, no glue is to be found, but Aang discovers a non-lethal way to stop Ozai, taking his bending and leaving him powerless.
Cucumber Facial: Katara and Toph's story in "Tales of Ba Sing Se" has them get one, complete with mud face mask. Toph earthbends the mud pack to look like a monster and scare one of the spa workers.
Culture Chop Suey: Sokka carries a boomerang, despite his water tribe culture being based off of the Inuit people.
The entire series is a mixture of cultures, including but not limited to Chinese, Japanese, Tibetan, and Inuit culture. The notion of the Avatar comes from India and the Hindu religion. There is also quite a bit of American influence, as the show, following the style of Japanese anime, is an American animated series.
Culture Clash: Zuko's sensibilities don't entirely align with Aang's even after he joins Team Avatar. For instance, in order to make up for having wronged Katara in the past, Zuko offers to help her murder an old man (who killed her mother, but still). Aang himself is subject to this as his upbringing taught him that killing was absolutely taboo, meanwhile his friends, who've all grown up in a world that's only known a century of war, have no problem with killing in battle. Even before that in "Bato of the Water Tribe" Aang is discomforted by the Water Tribe's culture of using fur for decoration and warmth among other things that are different to him.
Less Curbstompy, but still pretty much one sided, Pakku vs Katara shortly previous. Justified since he's an Old Master while she's a largely self-taught 14 year old.
Quite balanced, but Ozai clearly had an advantage over Aang in the last fight until HE reactivated the avatar state, turning the tables, and Ozai actually using evasive techniques like Aang moments prior.
Ozai had the advantage only because he was fighting to kill Aang, and Aang didn't want to kill Ozai.
Toph versus the Boulder in the final round of Earth Rumble 6 - she quickly dispatches him in a matter of seconds. Later, Toph wipes the floor with Xin Fu, The Boulder, and all of her old adversaries from the Earth Rumble competition after they kidnap Aang and decide to sell him off to the Fire Nation, resulting in Master Yu proclaiming her "the greatest Earthbender he has ever seen." This might be because she's a total badass.
The entire first six minutes of "The Earth King". It's pretty much Team Avatar going up against the entire Earth Palace's Royal Guard of about 200+ soldiers, and absolutely defeating them without even slowing down.
Cursed with Awesome: Aang never wanted to be the Avatar, and ran away when the monks tried to separate him from Gyatso.
Jeong jeong feels this way about firebending, claiming that firebenders destroy themselves trying to control themselves. He laments that he is not a waterbender instead.
Dance Line: in "The Headband", Aang is trying to teach some Fire Nation kids how to relax and have fun and does this at his secret dance party.
Dangerously Genre Savvy: Azula is very cunning, and has no reservations about resorting to the most dirty, underhanded tactics available if it will help her win. Case in point: in the second season finale, she sneaks up behind Aang while everyone else is busy staring in awe at his Transformation Sequence and sends a bolt of lightning into his back. After Zuko blatantly lies to her, saying that there's no way he could have survived, she sees through the obvious lie... and is summarily proven right. And then she turned Zuko's lie back against him.
The Avatars are supposed to be told about their identity on their sixteenth. Aang didn't get that luxury.
Sixteen is also the age in which Northern Water Tribe girls become eligible for marriage, much to poor Yue's dismay. It is unclear whether the same custom carries on to Katara's tribe. Though it seems unlikely since Kanna (Gran-Gran) ran away to the Southern Tribe specifically to avoid her arranged marriage.
Dark Action Girl: Azula and Mai. Ty Lee is a more downplayed example, in that she's a bad guy and an action girl, but her personality is anything but dark.
Darker and Edgier: The series as a whole is much darker and more serious than one would think for a daytime Nickelodeon show. There is direct mention of people having died before the series, a handful of people who die during the series, multiple mentions of mass genocide (strictly along racial lines). There are also rather mature themes throughout, such as parental abuse, murder, scandals, and having one of the main protagonists nearly kill someone of their own free will.
Deadpan Snarker: Several characters fit this one, including Sokka, Toph, Katara, Azula and of course Mai.
Even Zuko gets in on it at times:
Zuko: Yeah, and then you can show [the Fire Lord] his baby pictures and all those happy memories will make him good again. Aang: Do you really think that will work? Zuko: No!
Deadly Dodging: several examples, like Aang vs Zhao in "The Deserter", but the one taking the cake would be that scholar being attacked by the platypus-bear in the beginning of "The Fortuneteller". Aunt Wu wasn't that far off.
Death Faked for You: Iroh claimed to have killed the last dragon, but he instead found them and didn't tell anyone, so they could live in peace.
Also Aang, sort of. He actually does die but is healed by Katara and remains unconscious for some weeks after. The rest of the team think it's best that they don't draw attention to the fact that Aang isn't dead and eventually convince him to reluctantly go along with it
Declarative Finger: Zuko had one when he was impersonating his Uncle giving him sage advice.
Zuko: How am I supposed to convince these people that I'm on their side? What would Uncle do? [Impersonating his Uncle, pacing and holding up his finger] Zuko, you must look within yourself to save yourself from your other self. Only then will your true self reveal itself. [Dropping the impersonation and getting frustrated] Even when I'm talking for him I can't figure out what he means!
Deep South: All the swampbenders have hillbilly accents and seem adamantly opposed to wearing pants/shirts/shoes.
Defanged Horrors: This show has many scenes and creatures that are scary even to adult viewers, but stay clear of blood or violence, or monsters jumping at you from the screen. It also portrays a century long war quite realistically and does not shy away from discussing (and within limits showing) genocide and war crimes against civilians, while still remaining safe to watch for children.
Prisoners never seem to be executed. It's claimed by one Earth Kingdom soldier that the Fire Nation sometimes dresses prisoners of war in Fire Army uniforms and sends them unarmed to the front lines, essentially executing them by friendly fire, but it's never made clear if this is actually true.
Defense Mechanism Superpower: The Avatar State is explicitly identified as a defense mechanism by Roku, though it's also possible for an experienced Avatar to enter it at will. Aang doesn't learn this over the course of the series. Well, he tried in the second season finale, but that didn't go so well.
Defeat Equals Friendship: Iroh wipes the floor with a would-be mugger, then has a friendly chat and a Spot of Tea with him, kindly persuading him to take a new course in life.
Ty-Lee joins the Kyoshi Warriors after the fall of Ozai
Deliberately Monochrome: In "The Siege of the North" arc, when the moon spirit is killed, everything becomes black and white, except for Yue's eyes or when lit up by light from fire or water bending.
Demoted to Extra: Teo, The Duke, and Haru in late Season 3. At the end of "Day of Black Sun" they escaped on Appa with the rest of the Gaang... according to the commentary, because the writers felt that sending children to a Fire Nation prison would be too harsh. But the writers couldn't figure out anything else to do with them, despite Teo being a great glider pilot and inventor and Haru being a decent Earthbender in his own right… so they only appeared in the background, or the focus of "road trip with Zuko" episodes was set away from the Western Air Temple where they were encamped, until they could be Put on a Bus (along with the just-rescued Hakoda) in "The Southern Raiders". (In later commentary, the writers joked about wanting to do an episode entirely about their adventures exploring the Western Air Temple.)
Destroy the Abusive Home: Played with. Zuko brings things from the house to burn in the episode where he, Azula, Mai and Ty Lee go to Ember Island. The house gets destroyed properly some episodes later as he attacks Aang in the process of training him in firebending.
In Aang's fight with Ozai, when Ozai smashes Aang into a rock that hits him in the exact right spot to unblock the chakra that had been fried since Azula shot him with lightning, which allowed Aang to enter the Avatar State. Before that, Aang was losing and solidly on the defensive, but afterwards it turned into a Curb-Stomp Battle in his favor.
Rightly or wrongly the Lion Turtle's contribution to the Grand Finale, specifically Energybending, is sometimes accused of being this. However, it doesn't actually meet the requirements of the trope, as it doesn't solve an unsolvable problem. It just provides a third option because of Aang's Thou Shalt Not Kill beliefs.
Desert Bandits: The Sandbenders work as thieves out in the desert. One group stole Appa and sold him to a Fire Nation circus.
Sokka, of all people, gets this moment defending the Gaang from the beserk Owl Guardian in "The Library".
Sokka: That's called "Sokka Style." LEARN IT!
Die or Fly: Used twice. Toph Invokes this by rolling a boulder at Aang in an attempt to get him to learn earthbending. Aang jumps out of the way to keep from being squished. The scene is mirrored later in the episode, when he has to defeat an angry Saber-Toothed Moose Lion, and successfully gets the "stand your ground" mindset required for earthbending. Season 3 has a semi-straight example: Katara learns Bloodbending on her first attempt to stop Hama from making Sokka kill Aang.
In The Cave of Two Lovers, Aang jokingly says that he wouldn't want to kiss Katara. When she understandably takes it pretty bad, he "clarifies" that if he had to chose between death and kissing her, he would chose the kiss.
Aang: It was a compliment! Katara: Well, I don't know which I would chose!
Zuko lets drop that he was the one who sent Combustion Man after them, which he thought they had figured out already. Oops.
Katara in The Runaway while defensively asking Aang if she acts motherly, demands, "Stop rubbing your eye and speak clearly when you talk!"
Diminishing Villain Threat: Zuko as Season 1 progressed. He hadn't even become all that conflicted about what he was doing yet, but he kept on failing in his endeavors. No wonder Zhao wound up the final enemy of the season, despite Zuko being it's main villain.
It's also somewhat ironic since Zuko got noticeably and progressively more powerful throughout season 1, It just became more and more apparent as the season went on that he had inadequate resources facing effectively impossible odds. From the very first episode, the only reason he was a threat to Aang at all were his cunning, desperation, and absolute refusal to give up.
Toph's enhanced senses and earthbending prowess as a result of her blindness.
Often lampshaded and parodied with lines such as "Your feet need their eyes checked", "I'll tell your feet what's going on", and "Not being able to see with your feet stinks!"
Also, the Mechanist's son Teo is paraplegic, but has a glider attachment to his wheelchair that lets him fly almost as well as Aang. Though unlike Toph, this flight ability is never overtly suggested to be because of his handicap.
Disc One Nuke: The trope is toyed with after Aang finds out about the coming comet; he ALMOST learns fire-bending (sequence breaking the order he is supposed to learn the elements). He's actually quite powerful with just a small bit of instruction, but after he loses control and hurts Katara, both he and his teacher decide he is not ready.
Drawn to the forefront when they steal Fire Nation clothes, she punches her feet through the shoes to remove the soles.
This bites her in the ass (or the soles, if you will) just after Zuko's Heel-Face Turn (like, right just after), when she spooks Zuko at night in the woods, then doesn't respond when a wary exile with no one in the world he can trust calls out "who goes there". Predictably, the lack of response makes Zuko assume he's under attack and try to scare off his attacker by scorching their boots; boots which you may remember Toph doesn't wear.
In a broader example, the fire nation in particular evokes Japan in the years before and during World War II, and the cult of personality surrounding the Fire Lord has echoes of China under Chairman Mao.
Double Consciousness: Being both the son of the Fire Lord and the destined ally of the Avatar has got to be tough.
Don't forget being the great-grandson of both the man who started the war and the first to speak out against it (that is, the past life of the person he worked years to capture).
Double Edged Answer: Toph tries to explain how her feet got burned: she surprised and startled Zuko in the middle of the night and he reflexively flung fire at her before he realized it was one of the Gaang. He was immediately contrite and apologetic, but Toph, in pain, bailed.
Toph: Well, he did and he didn't.
Downer Ending: Several episodes of Season 2, most significantly the Season 2 finale.
Also, an in-series example: The ending of the Ember Island players' play (where the Avatar is killed, the Fire Nation wins the war and Ozai and Azula take over the world) was a downer ending for the Gaang.
The Dragon: Princess Azula to her father, Fire Lord Ozai.
Drama Bomb: The second season finale. There had been a gradual build up of conflict and drama in the episodes leading up to it, but it all came to a head in that episode with the force of a nuclear explosion.
Dramatic Thunder: First type in "The Storm" and second type "The Southern Raiders"
Early Installment Weirdness: Without getting into the differences in characterization, the pilot episode gives Zuko the ability to use flame jets as Reverse Gripped daggers and has Aang consciously activate the Avatar State for a quick boost despite the episode ostensibly taking place sometime during the equivalent of the beginning of the first season. Fire Nation soldiers wear Spikes of Villainy galore and Katara is also named Kya.
On the other hand, pilot episodes tend to be like this, and given the differences, the canonicity of the original pilot is questionable at best. (Also, Zuko uses the reverse-gripped daggers in Book 2's "The Avatar State").
Easily Forgiven: Averted. Most of the Gaang won't forgive someone's past crimes without a decent reason.
Special mention goes to Katara. Betraying her in any way will set her default reaction to you to "near-homicidal rage" and it will stay that way unless serious amends are made. The girl can hold a grudge.
However, played straight Iroh when the Gaang meets him in Ba Sing Se. Though Toph's testimony probably helped.
Eat the Dog: Sokka offers up Momo to a sea monster as a "humble and tasty" sacrifice.
Elemental Absorption Although not considered an element in universe, redirecting lightning surely counts.
Elemental Baggage: Averted; firebenders are the only benders who can "create" their element. Benders from the other three nations require the appropriate amount of material in their immediate area to be useful, and it's even made a plot point at times when a bender (specifically earthbenders and waterbenders) are rendered powerless by being deprived of their element.
Elemental Nation: All of humanity is divided into three distinct cultures based on the elemental powers of the inhabitants: the Fire Nation (plus its colonies), the Earth Kingdom, and the Northern and Southern Water Tribes. The fourth, the Air Nomads, were wiped out prior to the events of the show.
Embarrassing Old Photo: Aww, look at baby Zuko! Subverted and used for massive Mood Whiplash when the old portrait they found turned out to be of Fire Lord Ozai. That cute little baby grew up to be a monster
Enemy Mine: "The Blue Spirit" saves Aang from Zhao because neither wants Zhao to take him back to the Fire Nation.
Epic Fail: Hahn, a conceited water tribe leader, uncovers his disguise and charges at Iroh and the Fire General Zhao on their ship. What happens? Iroh and the General Zhao just step out of the way, Zhao easily flings him overboard, and Hahn falls in the water. Then Iroh and the General resume their merry war conversation as if nothing happened.
Very funny that he couldn't even learn his name. Calling him "Admiral Chao".
Bumi wears his robes all the time, unless he's showing off how great an earthbender he is.
For the Fire Nation, the fancy robes are often war armor, so it often makes sense to wear them a lot.
Also justified for Bumi, since his basic political strategy relies on convincing people he's a demented old figurehead-until suddenly it's time to show them that he's actually a demented old earthbending master, and also replace "demented" with "alarmingly savvy".
Escalating Brawl: Subverted. Hakoda tries to cause a prison riot by shoving the stereotypical huge tattooed guy... who responds with words because he's working on controlling his anger.
In the episode, "The Boiling Rock", Azula starts to lose it after Mai and Ty Lee do their Heel-Face Turn.
Also, in the episode, "The Avatar and the Fire Lord", Sozin left Roku to die on a volcano after helping him contain the eruption, around the time it was clear Roku had inhaled too much of the fumes to escape on his own. The look of betrayal on Roku's face says it all.
Iroh in the episode, "The Crossroads of Destiny", feels this way after Zuko betrays him for Azula and Ozai's approval.
Even Evil Has Standards: in "Zuko Alone", Zuko is starving and sees a camp set up by the side of the road, with a roast on the fire. He reaches for his dagger. He then sees that the camp belongs to a young refugee couple, the wife of which is heavily pregnant. He takes his hand off his dagger and leaves.
In his backstory, the incident that resulted in his scar started because he objected to sacrificing a division of new recruits for tactical expediency, despite not necessarily opposing the war.
Exposed to the Elements: Averted with Sokka and Katara, who wear heavy fur coats at both poles. Played straight with Aang, who never seems to be cold even though he always wears the same outfit (though this is Hand Waved by him knowing a special breathing technique). Averted and Justified with Zuko, who is seen using a special technique that allows him to breathe fire. He also dresses appropriately and is in very real danger of freezing to death after he gets knocked unconscious, prompting Aang to take him with them.
Zuko served as this to Katara in Season 1. Both lost their mothers at a young age, lost contact with their fathers for several years, and were forced to grow up very quickly as a result. Also, both have serious anger issues. Even their families are a twisted mirror of each other: a distant father, a lost mother, and a sibling who is eager to fight. Adopting each other's strengths in order to overcome their individual weaknesses helps them to grow as people.
Zhao was the Eviler Counterpart of Zuko in Season 1. Both favored power over control with their firebending and obsessed with gaining honor and reputation. Zhao was so obsessed though that it actually led to his Moral Event Horizon.
Similar in effect to Jet, Hamma served as this toward Katara; harboring deep hatred towards the Fire Nation because of what they did to her in her childhood. Hamma's unwillingness to let it go led to her becoming no better than the ones who persecuted her.
Azula became this toward Zuko in Season 3. Both suffered Parental Abandonment, although in Azula's case, it may only be from her perspective. Zuko however was able to get a Parental Substitute in Iroh while Azula was shaped by other parent, the super-evil Ozai.
For the ultimate example: Sokka, Katara, Hakoda and Kya versus Zuko, Azula, Ozai and Ursa. Their whole family is a counterpart: the older brother struggling to live up to his father's position, but overshadowed by the talents of his little sister, the bending prodigy; meanwhile, both siblings are still affected by the loss of their mother several years ago. Except Zuko's father is an unpleasable psychopath who exiled him, while Sokka's father is wise, kind and had to leave Sokka behind; Katara is hot-tempered but a total Team Mom, while Azula is cold-hearted and controls people with fear; and Kya sacrificed her life to protect her daughter, while Ursa murdered an old man in cold blood to protect her son (though, she also had to give up her life too, in the form of banishment, so she probably serves more as a Shadow Archetype).
Evil Is Burning Hot: The Fire Nation is initially played up to be this, but it's later subverted in that some become allies to Aang.
Evil Me Scares Me: Aang actually says, "I was scary" when reflecting on the Avatar State. A little different since the Avatar State isn't evil but rather an implacable force of nature.
Evil Old Folks: Fire Lords Sozin and Azulon, though the former came to regret his decisions and at least believed (or convinced himself) he was working for the good of the world. Ozai may or may not count, depending on how old he is. A few other Fire Nation villains are a respectable age too. Non-Fire Benders include Old Lady Hama, the vengeful and deranged Water Bender who abducts innocent Fire Nation civilians and locks them in an underground cave on the basis that they were born in the wrong country.
Evil Prince: Ozai was a classic example of this trope before he became Fire Lord. Subverted with Zuko, who ends up trying to overthrow his father not to seize power for himself, but to end the war. Azula is a complete aversion of this, because despite being evil, she remains completely loyal to her father.
Eviler than Thou: Zuko was plenty threatening during the first season, but Azula is superior in just about every way (except, you know, morals). She also manages to completely outmaneuver Long Feng in the Season 2 finale.
Exhausted Eye Bags: Aang, Katara, Sokka and Toph all have bags under their eyes when they go without sleep due to being chased for days by Azula and her Quirky Miniboss Squad. Later Aang gets this when he goes three days without sleeping due to nightmares of the coming battle.
Expository Hairstyle Change: Most major characters went through hairstyle changes over the course of the show, in stark contrast to many cartoons' habit of keeping hairstyles exactly the same to make it easier to animate. There were so many hairstyle changes, in fact, that it gave rise to the fan term "hairbending".
Zuko's hair changed from an exotic, samurai-inspired shaved-with-a-topknot look in Season 1 to a shaggy mop by the end of Season 2 while his character becomes less and less the villain and more and more three-dimensional. Said mop was thus long enough to tie back into a dignified Fire Nation topknot just in time for Zuko's coronation.
Katara's hairstyles in Season 3 were more elaborate than the ones she wore in Season 1 and 2, making her appear more mature… matching the way she grew as a "team mother" over the course of the show.
For the first two thirds of Season 3, Aang actually had hair (having been too busy being unconscious to shave, and determining it made a good way to disguise his head tat while sneaking around in the Fire Nation afterward).
Expy: Say, that guy who keeps losing arm wrestling matches and subsequently getting beat up by June... Does the spikey brown hair and tattered white gi with a red hachimaki remind anyone just a little bit of Ryu?
Face Palm: Sokka was so annoyed by the constantly singing nomads in "The Cave of Two Lovers" that at the end of the episode, this was asked of him:
In a memorable moment in Season 2 there was a sick Zuko in bed sweating and shirtless begging for water, when Iroh finally gives him a taste, he then takes the container and basically pours it all over himself. Does This Remind You of Anything?.
Any scene of Aang and Katara practicing waterbending will have Katara in a loincloth and bandage-like wrappings for her breasts and hips/thighs. Made even more fanservicey when you realize this is probably her underwear.
Fantastic Naming Convention: The different nations typically have names that follow phoneme patterns. The Water Tribe uses hard K sounds and lots of O's and A's (Katara, Korra, Noatok, Hakoda). The Fire Nation uses Z's, O's and I's (Ozai, Zhao, Sozin, etc). The Air Nomads use mostly real life Tibetan and Chinese names or names that sound similar. The Earth Kingdom has no strict pattern.
Fantastic Racism: The Fire Nation is utterly convinced that they're superior to all other peoples.
Air Nomads: Tibet, some elements of the Shaolin Temples of China.
Water Tribes: Inuit, with a little Pacific North West Indian (Northern Tribe).
Earth Kingdom: Qing (Manchu-dominated) Dynasty China with a little bit of Korea (Song's family from "The Cave of Two Lovers", Ba Sing Se's...unique worldview) and Japan (Kyoshi Islands) in the mix. The Sandbenders appear to be Middle-Eastern Bedouin, or — more likely — Muslim and/or Buddhist tribesmen from the Gobi Desert.
Peasant Clothing in the rural parts of Earth Kingdom is predominately Korean hanboks.
Fire Nation: Tang Dynasty China/Imperial Japan/Mongolia. There are also Indian touches (agni kai, for example). Finally, their war against the rest of the world with fervent nationalism and technological prowess calls back to World War II era Japan.
The original fire-bending civilization, the Sun Warriors, borrow from Native American cultures, specifically Aztecs and Mayans, but are predominately based on the Candi Sukuh of Indonesia, with architectural designs coming from Angkor Wat and Phanom Rung and headdresses resembling those of Iban warriors.
Fantasy World Map: The official Avatar site at nick.com has a nice world map documenting the Gaang's travels during the series.
Fate Worse than Death: Zuko and Iroh, while on the run as fugitives of the Fire Nation, were left with a choice: seek help in the Earth Kingdom, which mostly views all firebenders as war criminals and would likely be killed; or return to the Fire Nation, where they would be captured by Azula. Zuko quickly decides on the former.
Ozai's receiving Cruel Mercy can be considered one after losing his ability to firebend.
Jet tries at one point forcing Zuko to use his firebending so the people of Earth Kingdom would realise he's a citizen of Fire Nation hiding in their country. The reason why he knew Zuko to be a firebender in the first place is because his uncle used firebending to warm up his tea.
Sokka: Come on, a day at the theater? This is the kind of wacky time-wasting nonsense I've been missing!
Not abundant in the first two books, but jarringly common in Book 3. It's especially odd feeling because many filler episodes are put right in the middle of what would normally be the darkest arc of the series (or even worse, right before the finale like 'Ember Island Players').
Also Zuko and Katara; she comes to forgive him after he helps her find her mother's killer.
Momo and the three alley cats in "The Tale of Momo" from "Tales of Ba Sing Se."
Fire/Water Juxtaposition: Not surprising, considering this show's setting and mythology is practically built entirely on the Four Element Ensemble trope. Specifically, the first protagonists introduced in the show hail from the peaceful and simple Water Tribe, while the main antagonists are from the despotic and warlike Fire Nation.
The Avatars themselves often encounter this as well. Fire-native Avatars often find learning Waterbending to be the most difficult bending art to learn, and vice versa for Water-native avatars.
The Big Guy: Toph, although not really big, she fills the role well due to her strength and skills. Appa also counts as The Big Guy, at least before Toph joined.
The Chick: Katara, as the Team Mom and tries to keep the group together when there's tension between them.
The Sixth Ranger: Zuko, after two and a half seasons of chasing down the Avatar and working on his Character Development, he officially joins the group halfway through Book 3.Suki also counts, for making several appearances throughout the show, and finally joining the group halfway through Book 3. Suki also counts as a Guest Star Party Member.
Flynning: A deliberate, justified case on the part of one character. In Sokka's Master, Sokka and Piandao have a swordfight that lasts about three minutes. on the DVD Commentary, Sifu Kisu (the martial arts director) says that if this were a real swordfight, it would last under five seconds, but Piandao is just testing Sokka.
Foe-Tossing Charge: In the second season finale. After Aang is shot with lightning and is falling to the ground, Katara rides a gigantic wave over Zuko and the Dai Li to catch him.
Foreshadowing: In "The Storm" alone, we have Zuko caring about the lives of his people more than his father, Master Gyatsu messing around with Aang with a White Lotus tile, and Iroh redirecting lighting. Bear in mind this is halfway through Book 1: Water. Also, that blimp that gets shot down in "The Northern Air Temple"? The Fire Nation takes its tech to build a much larger one in the finale. An extremely subtle example; in "The King of Omashu", Bumi warns Aang that defeating Ozai will take extremely out-of-the-box thinking. See The Cloud Cuckoolander Was Right, above, for how thinking like a mad genius helped Aang in his adventures.
Zuko's dream in "The Earth King" can be interpreted as this, illustrating having to make a choice between his uncle, the red dragon (good), and Azula, the blue dragon (evil) two episodes later.
After running away from the pressure of being the Avatar, then being frozen for 100 years, Aang awakened in front of Katara. At first, he fell in Love at First Sight and all he wanted to do was play games or show off to her, all the while unaware of what he should be doing. Hanging on her every word, he gets on the right track in the Southern Air Temple. He finds friendship and camaraderie all around him, and comes to love the world selflessly. But it's always Katara he loves the most.
Zuko hunts the Avatar all around the world for his father's affection. When he returns to his father in Book 3, he not only regains a sense of belonging, he enters a happy relationship with Mai. But he later realizes his uncle Iroh loves him more than his father ever would. In the end, he comes to love and respect all four nations, is reunited with Iroh, keeps the Gaang as his first true friends, and gets to stay with Mai.
Free-Range Children: A Deconstruction. The kids have free reign to go on adventures because, with the exception of Toph, their parents are either dead or busy fighting in the war.
In Season 2 the Gaang starts getting chased by Zuko's much more competent sister, Azula, and her two friends who can throw knives and block bending. Then they make enemies with the government (Dai Li) of Ba-Sing-Se and the Fire Nation captures the city by the end of the season. Then in Season 3, the Fire Nation successfully captures the entire invasion force.
You could begin with season 1: in the first episodes, the plot is: the only hope to restore peace is a young, untrained boy. But a few episodes later, we learn that in a few months, the bad guys' powers will be multiplied by 100 for enough time for them to destroy all remaining opposition single-handedly.
Speaking of season 2's finale; when Aang gave up his love for Katara in order to save her by activating the avatar state, his most powerful trump card. And is shot down almost immediately by Azula, nearly rendering him worse than dead.
Or how they introduce the Series Finale? Zuko revealling that the plan by his father upon the advent of the comet is to burn the entire Earth Kingdom to the ground, destroying all life in an attempt to claim the land as permanent Fire Nation Territory. Making the whole "Stop the Fire Lord" thing a whole lot more serious.
Fruit Cart: The Cabbage Merchant, who manages to be the Fruit Cart regardless of where the main gang go.
Lampshaded in a couple of episodes where he mentions that the new city is worse than the last place the gang encountered him.
The Mechanist from "The Northern Air Temple" and "Day of Black Sun" is a classic lift right out of the Steampunk genre, a highly eccentric genius with steam and mechanisms who reluctantly lends his talents to making weapons for the Fire Nation when they threaten his home and people.
Sokka also shows some signs of inventive talent while at the temple and the submarines from "Day of Black Sun" were also his idea, and his father invented the "stink and sink" mine.
Hei Bei is a localized example. After the Fire Nation burned down an entire (assumably ancient) forest, the guardian spirit flies into a rage and terrorizes the (innocent) population of the countryside until the Aang shows it the acorns that will regrow the forest in time.
While not exactly a gaia example, the ocean spirit is one. When Zhao kills the moon, the ocean spirit becomes so enraged that it fuses with Aang into a monster, and after completely decimating the entire Fire Nation fleet attacking the North Pole, it's last act is to hunt down Zhao and drag him under. One of the rare instances in the show, especially the first season, where death is completely implied. Zhao angered the ocean, and it drowned him without mercy.
Game Changer: Learning about Sozin's Comet and the utter destruction the Fire Nation could use it for was a game changer in that it gave the heroes a limited time frame to accomplish the premise of the series, learn the four elements and defeat the Fire Lord.
Gecko Ending: An in-universe example in the Show Within a Show / Recap Episode "The Ember Island Players". In the episode the events of the series are retold, Abridged Series style, in the form of a play put on by a group of Fire Nation actors. Since this takes place before the series finale the characters don't know what the end of the story is going to be, so the playwright makes up his own climactic ending, wherin Zuko and Aang are both killed and the Big Bad wins.
Genius Bruiser: Combustion Man could be a bit. He is plenty strong and knew that a metal cage wouldn't hold Toph (as she is a metalbender, something unheard of before, and not universally known of her).
Genocide Backfire: Sozin's extermination of the Air Nomads in order to prevent the Avatar from reawakening there.
Genre Savvy: Displayed through a learned experience that Aang deals with in-universe in "The Firebending Masters". As Aang and Zuko (who has inexplicably lost his ability to Firebend) travel to an ancient Firebending temple that Zuko believes it might contain someone who can teach him a new way to firebend without resorting to rage, Aang accidentally triggers a booby trap concealing a huge row of spikes, just barely avoiding it with some nifty Airbending techniques, and Zuko, with some fancy Parkour. After acknowledging that they have to be more careful, the two continue to head deeper into the old ruins, coming across a shrine of statues (which they, due to the statues' positions, make the two believe might contain some powerful firebending techniques that were long forgotten) that signal a dancing pattern that is used to to reveal a sunstone on a pedestal upon completion. Upon the revelation of the sunstone, which Zuko believes might help them, Aang, still remembering his experience with the spike pit, cautions Zuko, stating that he just feels a little wary of glowing gemstones sitting on top of pedestals in the middle of a seemingly empty room.
This is how Toph grew up. She lived in luxury and had the run of the whole estate. But she wasn't allowed to travel outside the estate or exercise her incredible potential at Earthbending—her parents thought this was too dangerous for their "helplesslittleblindgirl." Nobody other than her family and her Earthbending teacher even knew that she existed, though she was able to get out enough to lead a secret double life as a pro wrestler
The Gaang's experience in Ba Sing Se. They were allowed to indulge in all the luxury they wanted, as long as they didn't try to leave, or break the rules, or evade the constant surveillance, or search for Appa, or tell anyone about Long Feng's Government Conspiracy or the war with the Fire Nation...
Subverted with Meng, to whom he doesn't give the time of day.
Giving Up On Logic: Sokka never fully does this, but he starts off as almost a Flat Earth Atheist who says that bending is magic and a flying bison could never exist to casually accepting all the various genuinely supernatural experiences the Gaang goes through.
A God Am I: While Sozin was more or less an Evil Overlord and nobody saw enough of Azulon to know what went on with him, Ozai fits this during the finale, just by crowning himself Phoenix King before he's even won. The hammiest example of this, however, was Zhao at the North Pole:
Zhao: I am... a legend now. The Fire Nation will for generations tell stories about the great Zhao, who darkened the moon! They will call me Zhao the Conqueror! Zhao the Moon Slayer! Zhao, the INVINCIBLE!
Gone Horribly Right: In the Season 2 opener, General Fong convinces Aang to attempt to use the Avatar State to take on the Fire Lord without having learned all the elements. After many failed attempts of bringing on this state however, Fong grows impatient and tricks Aang into the Avatar State by pretending to harm Katara. He gets his wish alright, and a good chunk of his fortress is destroyed in the process.
Good Angel, Bad Angel: Zuko's uncle and sister in the Season 2 finale; in the episode before, Zuko goes into an Angst Coma and dreams about a Red dragon that speaks with Iroh's voice and a blue dragon with Azula's. This is a neat foreshadowing of Avatar Roku and Fire Lord Sozin being his implied historical and psychological Good Angel, Bad Angel in The Avatar and the Fire Lord - Roku (Ursa's grandfather) owned a red dragon and Sozin (Ozai's grandfather) owned a blue-green one.
Good Cannot Comprehend Evil: Aang holds this view for the majority of the first season, in keeping with his pacifistic views. Even at the end, when he's facing Ozai, he can't bring himself to kill him.
Zuko's scar shows that the writers and character artists are well aware of this trope.
Aang's eyes are by far the widest of all the characters.
By Season 3 Zuko's unscarred eye become wider and less menacing.
Good Old Fisticuffs: Sokka has no bending abilities and had little to no formal combat training, but he can generally hold his own in a fight.
Good Scars, Evil Scars: Zuko's facial burn actually covers 2 variations of this trope. When he is evil, it makes him look that much more menacing, but in the episodes where he is good(ish), it makes him look more noble — it is actually taken as a cue by Song and Jet that he is a fighter against the Fire Nation.
Averted. Benders of both genders freely mix long- and short-range attacks, as does Sokka. Mai might only do ranged attacks, but Ty Lee and Suki are strictly melee fighters.
A more proper divider would be "Muggles Smash, Benders Shoot". It seems if you know any bending, then you can only fight at all with Bending—the only people who throw physical attacks that connect are the non-benders. Any bender will only ever make contact with an enemy with their element except, occasionally, to block.
Except Zuko, who takes out a few Earth Kingdom soldiers without bending in "Zuko Alone" not to mention his escapades as the Blue Spirit
Coinciding with the above statement, although Muggles Smash, Benders Shoot is the better descriptor. In terms of the Gaang this really only applies to Katara who is not at all versed in hand to hand combat and really is at a bad disadvantage when she can't bend or has no water to draw from. Generally however, the more talented benders seem to be rather capable at handing out beatdowns or at least getting by without their element; Zuko [as noted above), Aang (a rarity, but can go on a really good dodging defensive), and Iroh for sure and highly possible for Azula and Toph.
Handsplay in Theater:(regular theater, here)in the episode "Ember Island Players". Played straight with Sokka and Suki (his arm around her shoulders), and averted with Aang and Katara (Zuko sits between them.).
Happily Ever After: Played straight in the show itself. The sequel comics show that the Gaang still has a lot of work to do and explores the issues that arise from the Fire Nation colonies.
Hard Work Hardly Works: Subverted. It looks as though Aang learns waterbending faster than Katara (which would at least be justified by being the avatar), but when Katara and Aang get an actual instructor instead of relying on self-teaching and the scroll, Katara masters it even faster than Aang. Furthermore, Aang has trouble with earthbending since that is the counter to air. Of course Aang is shown goofing off during the waterbending lessons.
For the brief moments we view Iroh in his prison cell, you gradually see him transform from a badass sack of lard into a even more badass chissled bodybuilder. This also parallel's Sokka's sword training.
And this was the creators response to Nickelodeon when they asked for an armor for Aang for commercial reasons.
He Who Fights Monsters: The series provides two examples of this in Jet and Hama, two individuals whose hatred of the Fire Nation blinded them to the point where they could no longer distinguish between friend and foe. Hama was even worse than Jet, because he at least tried to justify it as doing the right thing by making sacrifices, and he did redeem himself in the end. Hama was treated so badly that she targeted anyone within reach, mostly civilians. Both of them served as a warning to Sokka and Katara about what they could become if they continued to hold on to their own prejudice and anger.
Heart Symbol: In "The Boiling Rock". Sokka exhibits it when he realizes Suki is a prisoner there, so his mission with Zuko is not in vain, and he gets his Love Interest back.
Heel-Face Turn: If you watch the first season you will not be too surprised by Zuko turning. But it is played with and at least initially subverted in several instances, the second season finale being the most infamous.
The spirit Hei Bai is one of the earliest examples, though all it took was reassurance that his forest would grow back.
Heel Realization: Zuko spent the third season on this. But the Ember Island Play really hit him hard on how he treated the only person that really cared about him, his uncle Iroh.
More like a Jerk realization. After Pakku defeated Katara, he didn't seem to change his machist ways until he discovered that she was Kanna's granddaughter, and then Katara gave her speech, in that moment Pakku realized that were this Machist ways that costed him the love of his life.
Heh Heh, You Said X: Aang and Sokka snicker when the professor admires the buttresses in Wan Shi Tong's library.
Held Gaze: the series has a few of these. One with Mai and Zuko in the Boiling Rock episodes after being forced to lock the cell door on her [this one is more of deep hurt though], one with Suki and Sokka on the Serpent's Pass leading to an Almost Kiss. Then finally the one between Aang and Katara which leads to the Big Damn Kiss and end of the series.
High Fantasy: An epic that involves the fate of the world, a young boy and his Ragtag Bunch of Misfits who are Walking the Earth, politics that change the course of the world, gods (called spirits but act as deities) that interact with the protagonists and, as a refreshing twist, takes place in a mythical world insipired by Eastern culture rather than Western. It would go in Wuxia save for the fact that it holds very little in common with the genre.
High Voltage Death: Subverted. Azula uses lightning bending and strikes Aang in the back as he is entering into the Avatar state, threatening to end the avatar cycle for good. It at least looks like he died until Katara revives him with water from the Spirit Oasis, subverting a permanent version of this.
His Own Worst Enemy: When Aang truly mastered his full power by beating Fire Lord Ozai was very easy. However, most of the issues he faces are him accepting his role as the Avatar and stopping/running away for his duties. He becomes much stronger atfer he accepts his responsibilities. In fact, his greatest failure isn't not stopping the war, but running away from it.
Hope Spot: Happens in the second season finale when Aang enters the Avatar State intentionally for the very first time and looks ready to wipe the floor with everybody... only to be electrocuted by Azula.
The entire episode right before the last two episodes of the second season. Appa's back, the Earth King has sided with the Gaang, Long Feng has been arrested, and each of the characters have personal issues that are looking up. Unfortunately, Long Feng has allies and is planning in the background, Toph is captured, and Azula, along with Mai and Ty Lee, has disguised herself as a Kyoshi Warrior and about to show how good of a Chessmaster she is.
Humanoid Abomination: The Avatar State borders on this, especially during the fight with Ozai in the finale. Since the state is basically allowing all previous avatars to act through you instead of yourself, it's much more brutal and ruthless than Aang is normally.