Half-Human Hybrid: Apparently, the accident turned Danny into a "half-ghost", which is refered to as "halfa".
Hammerspace: On exactly one occasion, Danny is shown carrying the Fenton Thermos by a shoulder strap. The rest of the time, he just pulls it from behind his back, seemingly out of nowhere.
Arguably, this also applies to his costume while he's human, and to whatever clothes he's wearing when he transforms. While he was wearing his normal outfit under the jumpsuit when the portal first zapped him, whenever he was wearing pajamas or a suit prior to going ghost, that's what he was wearing when he changed back.
Hand Blast: While not really a thing ghosts usually do, Danny can fire ectoplasmic blasts or "Ghost Rays" from his hands.
Spectra: Look at you? What are you? A ghost trying to fit in with humans or some creepy little boy with creepy little powers?
Danny: Both! …Neither! I don't know!
Spectra:You’re a freak! Not a ghost, not a boy! Who cares for a thing like you?
Happily Ever After: The world is saved from a giant asteroid, Jack and Maddie (and arguably the whole world) knows and accepts Danny's identity as a half ghost, Danny is regaled as a hero around the world, the Big Bad has been banished from the planet, and Danny and Sam get together in the end.
Hartman Hips: Maddie, Valerie, Paulina, etc... it is a Hartman series...
"Man, that's hot!"
Sam in "Urban Jungle", but otherwise she's quite trimmed.
Heh Heh, You Said X: In "Pirate Radio", after Youngblood and his pirates swipe the Fenton Ghost Shield, Youngblood turns to his parrot for the term, to which he says 'booty'. Youngblood, his crew, and even Tucker giggled at the word.
Heroic Willpower: Displayed, surprisingly enough, by none other than Jack Fenton in Bitter Reunions. At the time of the episode, he was the only individual without ghost powers to ever stave off an overshadowing, even if for a short period.
Interestingly, he only managed to fight back against Vlad, who is evil, and remembered being possessed by him. When Danny possessed him, on the other hand, he instantly submitted and remembered nothing.
Hey, It's That Voice!: This show had not only a multitude of popular voice actors in its regular lineup, but often hired famous Hollywood actors to voice the villains. (Yep, even Undergrowth.)
Hidden Agenda Villain: Vlad: The purpose for getting the Skeleton Key isn't revealed until another episode - which in turn holds another hidden agenda, taking the Fenton Ecto-suit and the loyalty of the Fright Knight. While the first is dealt with, the show's abrupt end never showed the conclusion/purpose of the latter. Also fitting the ghost-hunting equipment he gives Valerie, the purpose of which we find out over a season later - that being giving him resources to clone Danny.
Hoist by His Own Petard: Happens to Vlad more often then not, but really comes to a head in the series finale when he makes a Batman Gambit to take over the world in exchange for saving them, revealing his ghost half to everyone in the process. This backfires horribly resulting in him being self-exiled from Earth and drifting in space, if not dead.
Also happens to Danny when Jazz accidentally sucks him into the Fenton Thermos in "Secret Weapons".
Homage: Multiple. The Goth Bookstore showcases George McFly's book from Back to the Future on its shelves. When Danny collapses from cold in "Urban Jungle", he wakes up floating in his underwear in a water tank a la The Empire Strikes Back. Then there's "three for the OpCenter" as they "beam up" to the upper level of the Fenton house. And those are just the really obvious ones. There's more than a few examples in dialogue as well.
To add into the "Back to the Future" references, "Masters of All Time" has a blink-it-and-you'll-miss-it" cameo of Marty McFly in the group shot of the 80's college student (wearing his infamous red "life jacket") and an even cooler homage: the clock tower on the school reads 10:04. Awesome.
Adding yet again to the "Back to the Future" references, the first reference is found in "Splitting Images", the fifth episode of the first season. Marty can be found this time as one of the kids in the 50s version of Casper High.
One of the reasons Sam Manson is considered The Scrappy is because of her hypocritical behavior. She says she opposes violence yet is fine with hitting her friends, namely Tucker, even when it isn't needed. She despises forcing beliefs and opinions on to other yet forced the entire school to eat her "recyclable organic matter" because "we don't need meat". She scolds Danny for using his ghost powers against powerless school bullies yet she's okay with causing millions of dollars in property damage to a dealership because they were selling eco-unfriendly trucks.
Danny's archenemy, Vlad Plasmius, may count as one too. Despite his claims that all he wanted was to be loved, he treats the only two people who showed him any kind of love or admiration like dirt all because they weren't the wife and son he always wanted. He instead obsesses over a woman, a married one at that, who doesn't even like him as a friend and doesn't care how she actually feels and gaining his teenage archenemy as a son.
Hypocritical Humor: Sam got upset with Danny for using his powers to spy on her and Gregor, but when Danny was spending time with Valerie, Sam wasn't exactly keeping her nose to her own business.
Impairment Shot: Danny wakes up to see Vlad and Dani out of focus in "Kindred Spirits".
Improbable Hairstyle: Many of the ghosts have this, but this is averted with most of the humans. Except for Danny. What kind of hairstyle is that, a reverse mullet?
Inconvenient Itch: In one episode, Danny and Valerie end up in the Ghost Zone and are forced to team up against Skulker. After a while, they end up trapping him, tying him up, and leaving him on the floor as they exit the zone. At that point Skulker exclaims:
Skulker: Err... guys, there is a problem... my nose is itching.
Danny: (noticing Box Lunch) A ghost? Here? (to Jazz) Now get out of my room! Jazz: We're in the kitchen... (also noticing Box Lunch) ... but if that's your attitude, then I don't want to see you ... or ... anything you do in here for the next ... several minutes! (exit stage left).
Infant Immortality: Averted in "The Ultimate Enemy" which shows the only cold-blooded murder committed in the series and the accidental deaths of Jack, Maddie, Mr. Lancer, Jazz, Sam and Tucker. Sidney Poindexter may also be an aversion, but he's already a ghost. An annoying one at that.
Informed Judaism: Actually averted, if you pay attention - Grandma Manson occasionally uses Yiddish words when she's otherwise speaking English, and Manson is a fairly common Jewish last name. Ironic since Sam's parents are characterized as WASPs.
From "Bitter Reunions", after Danny scares off the ghost vultures in the castle and first meets Vlad's ghost form:
Danny: I was aiming for the birds, but you'll do! Vlad: (mockingly, after Danny fails at attacking him twice) My vultures were supposed to bring the fat idiot to me, but you'll do!
In "Maternal Instinct", when Vlad first uses the Plasmius Maximus on Danny to temporarily De-power him:
Vlad: I've seen your grades, and I know you're bad at math. ... I'll give you five minutes before I send my minions to destroy you. (mere seconds after Danny flees Vlad's cabin) Get him! Five minutes, five seconds — oh, apparently, I'm bad at math, too!" (Later, after Danny gets the better of him and DePowers him with the same device): —>Danny: I'll give you a five-minute head start, Plasmius. Vlad: Really? Danny: Minutes, seconds... you know how bad I can be at math. (to Vlad's minions) Get him!
In "Shades of Gray", Valerie was planning to go to a concert with the other popular kids, but subsequently had to sell her tickets online to pay for a moving truck. Paulina then asks her who bought them ("We don't wanna have to sit next to any losers at the concert."). Later, it turns out that Sam did:
Sam: (at the concert with Danny and Tucker) I can't believe we're sitting next to those losers!
"Life Lessons" begins with a great example. When Danny is fighting Valerie, he says "I don't wanna hurt you," to which she replies "What makes you think you can?" Later, after finding out that they were assigned to the same Home Economics project, the teacher mocks them, referring to them as a married couple. Danny says "I am so not kissing the bride" to which Valerie replies, "What makes you think you can?"
As it happens, that episode has another instance of this trope; after they finally manage to beat Skulker and remove the handcuffs binding them together, Valerie apologizes to Danny (Phantom).
Valerie: I never thought a ghost could be anything but trouble. Danny: That's because you never bothered to get to know one.
And a few scenes later, when Danny (Fenton) finds out the job Valerie has...
Danny: Wow, this is like the worst job ever. I didn't know this is the job you were talking about. Valerie: That's 'cause you never bothered to get to know me.
It's Not You, It's My Enemies: Ironically, Valerie severs her relationship with Danny (Fenton) before it can start because she doesn't want him to get hurt because of her job hunting ghosts, including Danny Phantom.
Jerkass: Vlad, especially in the series finale, after he reveals his ghost powers to the world. Jack, while surprised at this, is willing to overlook it and try to rebuild their friendship. Vlad coldly blows this off however since he thinks his plans are coming to fruition. When they don't, though, he tries crawling back to Jack. Say the least, his chance at redemption is long since gone and Jack leaves him to rot.
A bit of Alternative Character Interpretation can cause the Ghostwriter to be this. He causes the whole world to torture Danny because Danny accidentally incinerated a Christmas poem he wrote. Sure, he considered it his Magnum Opus, but what self-respecting writer considers a Christmas poem his greatest work? And if it really was that important, he just could've used his power to rewrite reality and make it so that Danny never incinerated it. In order to torture Danny, he breaks an annual Christmas truce that EVERY ghost follows.
Jerk Jock: Dash and Kwan are insufferable that way.
Just a Kid: Danny gets this a lot. His in-universe nickname is "Ghost Boy". More notably, Vlad does this twice, once almost calling it out ("Bitter Reunions"), the other as more of an insult ("Reign Storm"). Clockwork also does this once, but it's justified - he's trying to explain the nature of time and his own existence to someone who's probably flunking high school math. ("Why am I bothering? You're fourteen.")
Valerie also remarks this about herself when her ghost-hunting benefactor reveals himself. "But why me? I'm, like, fourteen."
Kick the Dog: "Besides, shouldn't you be failing a test, kicking a puppy, or beating up somebody weaker than you now?"
Lawyer-Friendly Cameo: In "Eye For An Eye", we get Vlad overshadowing Bill Gates and selling himself Microsoft - no, pardon, Mastersoft.
An episode later, Vlad flips through channels, and mentions an "embarrassing celebrity arrest." Don't blink when the camera shows the tv, or you'll miss Michael Jackson.
In "Attack of the Killer Garage Sale" Technus' operating system is Portals XL, which seems to be a reference to Windows XP.
Least Rhymable Word: In the Christmas episode, everyone was forced to speak in rhyme as someone narrated the entire episode. That is, until Danny picked up an orange, prompting the Ghost Writer to remark irritably that nothing rhymes with orange.
Later, in Ghost Jail...
Walker: Orange? (offers it to Ghostwriter). Ghostwriter:GET THAT THING AWAY FROM ME!
Yet he still wears a purple coat...
Tucker believed that the reason Danny doomed us all by getting rid of his powers in the Finale was because nothing rhymed with Phantom.
Logical Weakness: Ever tried screaming really loud for a really long time? This is why the ghostly wail is only ever used by Danny as a last resort, as it completely drains him and deactivates his ghost form.
Love Makes You Evil: Story of Vlad's life. Losing everything he loved caused it to be Danny's future. Somebody hit the [[Reset Button]].
Which they did in "Masters of All Time". It got worse. Vlad and Maddie married, but Vlad has made her into nothing more than a bitter housewife and Jack a major woobie as a half-ghost shut in. But even then, Maddie and Jack loved each other in this future and helped Danny go back and change it the right way. In an interesting subversion of this trope, Vlad was still evil regardless.
Danny: I couldn't imagine my life without you. [beat] Or Tucker. [monster appears in the distance]] Stay here. Wake up Wulf.
Sam: Be careful. I couldn't imagine my life without you either. [beat] Or Tucker.
Loves My Alter Ego: Paulina idolized Danny Phantom and typically dismissed Danny Fenton. It later happens in reverse with Valerie, who loves Danny Fenton but hates Danny Phantom! Boy just can't win.
Made of Iron: Danny. Even putting it down to a ghostly Healing Factor, Danny gets shocked, blasted, and slammed into walls so often any ordinary superhuman would at least fracture something. Even disregarding the title character, the ordinary human characters survive at least crippling falls without even bruising.
Magic Mirror: The mirror in Poindexter's old locker acts as a portal between Caspar High of the present and a 1950's version of the school in the Ghost Zone.
The kids all attend Casper High. Can they really complain about the constant ghost attacks?
Also, they live in Amity Park, Amity being a reference to the Amityville Horror.
Kwan, a that's name Korean in origin, meaning "strong".
Some of the episodes use punny names to hint at villain reappearances like "Dr. Bert Rand" from the episode Doctor's Disorders or M. Bursback Cruise line in Pirate Radio referring to Bertrand, Penelope Spectra's assistant and Ember's second appearances, respectively.
Mercy Lead: Valerie in "D-Stabilized". Subverted in "Maternal Instinct" by both Vlad and later Danny.
Merry Christmas in Gotham: The show has an all-round Christmas Spectacular as Danny teams up with his enemies to fight someone who is trying to spoil the event. Unusually for a children's show, Danny loathes Christmas, but with good reason.
Mind Rape: Vlad's attempts to get Danny over to his side. Done later on when Vlad tries to remove Danny's human emotions (at Danny's request, chillingly). That it backfired is putting it mildly.
Mood Lighting: Seems to represent a drop in temperature coming with the presence of a ghost.
Motive Decay: Vlad's initial, intriguing motive to turn Danny into his son and apprentice, putting him on par with Slade from Teen Titans, disintegrated without explanation by Season 3, leaving him nothing your average, run-of-the-mill Villain with Good Publicity.
Possibly justified. Vlad is an extreme narcissist. Being repeatedly beaten or at least matched by a 14-year-old boy with over 20 years less experience with ghost powers than him is likely to eat away at his ego which would probably lead to an evaporation of any positive feeling or goodwill. The desire to destroy would easily overwhelm the desire to teach.
Mustache Vandalism: The first thing Technus does as soon as he's freed from Danny's old game of Doomed is to open a paint program with a picture of Danny's crush Paulina and draw a mustache and massive glasses on her. Later, once Technus beams himself into the mainframe of the Fenton RV, he does the exact same to a picture of Jack Fenton.
My Friends... and Zoidberg: Lancer in "The Ultimate Enemy." Jack, Maddie, Jazz, Sam and Tucker are all "gone, but not forgotten." Lancer is just "gone" due to his statue being half-sunken into the ground.
Myth Arc: Danny's coming of age as a superhero and struggle with his darkness, his deepening relationship with Sam, and the world's gradual acceptance of him - and that's just the main character.
Jazz's arc where she slowly and eventually accepts her parents' ghost hunting habits and eventually getting into the life style; she also learns to embrace her childhood instead of acting like an adult all the time.
Valerie's growth from a shallow, popular teen in the background to a fierce ghost-hunting warrior who starts to see a world beyond what she perceived while she was spoiled.
Vlad's continuous desire for Maddie and especially Danny that culminated in his Villainous Breakdown.
New Powers as the Plot Demands: Kind of justified since it serves as a more physical metaphor for his character growth (his powers grow as Danny himself does)*
New Superpower: After establishing the basic ghost-related abilities, Danny gets newer, less-ghostly, energy-based powers about every other episode. One notable gain occurred in "Urban Jungle", where Danny develops ice- and cold-generation powers just in time to fight a plant ghost who is particularly vulnerable to cold. However, at least the producers were classy enough to foreshadow this new power; Danny's ghost sense is said to be a by-product of it. Furthermore, the episode used that power as the springboard to show the deepening friendship with Danny and Frostbite as student and mentor.
The ice thing is also semi-justified in that ghosts are commonly associated with cold (see Mood Lighting). And Vlad specifically says that Danny shoots a beam of ectoplasmic energy.
Nice Job Breaking It, Hero: Danny gets manipulated into screwing up a few times. It almost always turns out alright and the one time it didn't found someone hitting the Reset Button. Ironically, he messed up all by himself that one time. It ended up destroying thousands, if not millions of lives... which made the Reset Button required.
Sam had a moment herself in "Memory Blank".
Clockwork attempts to prevent the Bad Future by sending a ghost to eliminate Danny. The fight between Danny and that ghost sets the events leading to that same Bad Future into motion.
Our Souls Are Different: On multiple occasions, Danny Fenton's soul is shown to be separate from Phantom. Originally happened via a forced "Freaky Friday" Flip with Poindexter, who was capable of morphing into Danny Phantom while in Fenton's body. Subsequently, any time Danny's ghost and human halves are split - either by the Fenton Ghost Catcher or Vlad's Ghost Gauntlets in "The Ultimate Enemy" - the latter is conscious and active.
Out of Order: "The Fenton Menace" and "Identity Crisis" aired after "The Ultimate Enemy" but were set beforehand.
Despite being set afterwards, "Double Cross My Heart" and "Kindred Spirts" aired before "Reality Trip".
In addition, "Urban Jungle" was the first episode of the third season to premiere, even though it was number six. The Season 3 premiere didn't even air in the U.S until after the following seven. Honestly, Nick didn't handle new episodes very well.
"If you upset my daughter, we are going to have a very violent talk."
Also, Valerie's dad, Damian, to a lesser extent. He's okay with Danny and Valerie being together, just so long as none of Danny's parents' "ghost weirdness" rubs off.
Painful Rhyme: Lampshaded in-universe by Danny to Lancer in "Teacher of the Year":
"In the poem, 'The Tyger,' why does the author rhyme the words 'eye' and 'symmetry'?"note To answer Danny's question, the letter "y" in British English during the 1800s always sounded like the "i" in the word, "kite."
Pair the Spares: "I am Box Lunch, daughter of the Box Ghost and the Lunch Lady!"
Once, when Danny began dating Paulina who really being overshadowed by Kitty, Sam got unwillingly paired up with Kwan while Tucker began dating Star. Finally, Star and Kwan hooked up with each other, allowing Sam and Tucker to both be single once more.
Post-Victory Collapse: Rather often, starting in the first episode with Danny saving Tucker and Sam from the Lunch Lady, only to collapse afterward. Subverted in the climax of "Ultimate Enemy". Eventually, he grows out of this as he gets used to his powers and develops them further.
Power Degeneration: The fate of the Danny clones, and almost that of Dani when overextending her powers
Power Dyes Your Hair: When Danny first gets his powers and, subsequently, whenever he "goes ghost", his hair turns white. In "Phantom Planet", when he decides to reverse the accident that gave him his ghost powers and get rid of them, he gets a streak of white in his hair, which goes away when he gets his powers back. It is even commented on in-universe.