Awesome Music: Say what you want about its lack of faithfulness to the original show's lovely musical style, but James Newton Howard created an incredibly beautiful soundtrack that tries its hardest to elevate the film.
Bile Fascination: Those who joined the Avatar fandom after the release of this movie can't help but be curious to know why it was panned to kingdom come.
The debate over the movie casting was so intense that entire forums were divided over it, with those supporting the casting and those against it both being banned for their opinions. Protesters considered those who supported the casting as blind and shrugging off a big issue, while those that eagerly anticipated the movie regardless thought the protesters were not "true" fans.
Firebending only being able to manipulate preexisting flames (except in special circumstances). People either considered it a nonsensical change that severely underpowers firebending, or felt that it now fits with the other three bending types in that it doesn't create the bendable element, only manipulating what already exists. note This is despite Iroh having explained in the show's first season that firebenders bend their body's heat into fire.
Dork Age: For Shyamalan, this movie was easily the lowest point of his career, having received universally bad reviews from professional critics and all corners of the Internet, along with earning a huge backlash for its unfaithful, soulless approach to its source material. It followed up on the critical failure of The Happening and for many (at the time) signaled the end of Shyamalan's dominance in Hollywood. However, after the similar critical failure of After Earth, Shyamalan was able to make a comeback with The Visit and Split.
Eight Deadly Words: One of the most fundamental criticisms of the film, beyond whatever liberties it took with the source material, is that it is generally seen as uninteresting and unengaging.
Francis Guinan as Pakku was noted by several reviewers to be the one actor to escape the film with his dignity intact, having put real effort and emotion into his scant screen time in stark contrast to everyone else.
Seychelle Gabriel's performance as Yue also was moderately well received by fans, and Bryan and Mike would later cast her as Asami Sato in The Legend of Korra.
Even though his portrayal may not have been entirely accurate to the character he was playing, most critics noted that Shaun Toub as Iroh was one of the few cast members who put effort into their performance.
Fan-Preferred Couple: The Zuko/Katara pairing was a popular ship for the fandom during the original series, and was also true for the movie. However, many of those who had only seen the movie gravitated towards Zuko/Katara because of their fight at the North Pole and the lack of any Katara/Aang teasing (as Aang is unable to have any romantic attachments).
Bending in the movie altogether is severely depowered. Instead of the elements moving with the Bender, the Bender preforms entire Kata "to build up Chi" before anything happens, which slows things down and sometimes makes it seem like it would just be better to just fist fight instead.
The ending of the Oasis fight was significantly more Zutara-ish.
Not to mention some of Zuko's interactions with Aang.
"Funny Aneurysm" Moment: In the movie, Gran Gran Kanna said "Once, a long time ago, the Spirit World kept balance over us." In The Legend of Korra, that pretty much meant the Spirits invaded the human world, spiritformed everything, and forced the remnants of humanity onto the backs of Lion Turtles. Or put another way, "Then, everything changed when the Spirit World attacked."
Germans Love David Hasselhoff: Claimed by the director. M. Night Shyamalan has claimed on several occasions that the film was better received in Europe, and has gone on to claim that American critics "don't get" his European sensibilites. There is, however, very little evidence that this is true, and Shyamalan has only offered personal anecdotes to support the claims that Europeans like the movie better.
Heartwarming Moments: The scene where Iroh gives Zuko advice on how to survive the freezing cold before Zuko leaves for the North Pole is genuinely touching, due to how well the actors acted in that scene, with Iroh showing genuine concern for Zuko's welfare. It's one of the few moments to feel like it could have come from the show.
"The Ember Island Players" episode, which came out during the production of the movie. Needless to say, many jokes were to be had using footage from that episode to show how bad the film was — particularly its last line, wherein the heroes say the show they just saw was awful but the effects were nice.
Seychelle Gabriel (Movie Yue) voicing Asami in The Legend of Korra, a character first intended as a simple Romantic False Lead but was changed to become much more prominent when she became popular with the fans. The hilarity doesn't reach its peak until the end of the show, though, where Asami ended up becoming the titular Korra's final love interest. Complete with the fact that Seychelle originally auditioned for the role of Katara, Aang's love interest.
Cliff Curtis later joined the sequels to Avatar, the film which gained the ire of Last Airbender fans for preventing The Legend of Korra (and this film itself, but not too many people cared about that) from using the word in its title.
A common trend on sites like Reddit is to pretend the film was never made (e.g. "I wouldve liked to see that movie. Too bad it was canceled"). A snowclone of this is to post the statement "There is no movie in the franchise. The Earth King has invited you to Lake Laogai," (or some variation upon it) whenever the film is mentioned.
Mis-blamed: M. Night Shyamalan was certainly not blameless in the quality of the final product, but it is now known that a lot of Executive Meddling robbed Shyamalan of a lot of the control he had on set, including much of the editing that led to the film being deemed an incoherent mess in theaters. Just a few examples of the issues he ran into:
Paramount didnt want to wait a year to let the show wrap up before they started filming, so there was a fair amount of writing in the dark. Shyamalan's original script, that was thrown out, is reported to more or less have stuck plot by plot to the show. The ghostwriter of the final script accidentally backed them into a huge corner plot-wise for the rest of the series. Specifically, Aang running away because he was told the Avatar couldnt have kids; Zukos whole character arc depends on him being the direct descendant of an Avatar.
The studio blew most of the budget filming the opening sequence in the Southern Water Tribe on location in Greenland. This caused the rest of the movie to be filmed inside and not look nearly as good by comparison. They then realized that they couldn't use as many practical effects as they had thought they could, adding to the budget woes.
Shyamalan didnt actually want to cast white actors to play Katara and Sokka. Kataras actress was cast because Paramount owed her father a favor. Other actors in the Northern Water Tribe were thus cast to match her appearance.
The studio also mandated that the runtime be under 100 minutes, making a lot of it incoherent. The short run time was mandated because they needed to rush to get it converted to 3D in time to meet the lucrative Independence Day weekend release. They also would have likely run out of money to convert the original cut of the movie.
The mispronunciation of the characters' names; for example, Aang being pronounced as "Ong", and Sokka is pronounced "Soh-ka". The fandom went to town with this and it quickly became symbolic of the film's faults.
Older Than They Think: A lot of people criticized the film for introducing the idea that Avatars cannot have romantic or familial relationships. However, a major plot point in the second season finale of the cartoon is that Aang's romantic attachment to Katara prevented him from entering the Avatar State, which implies that Avatars cannot operate properly if they are in romantic relationships. It's just that the show later ignored this concept and turned the whole thing into an Aborted Arc, while the film ran with it.
Katara's live-action incarnation had far fewer fans than her cartoon counterpart, mainly for hardly being like Katara is in the cartoon and her actress having a severe case of Dull Surprise. It also doesn't help that the script removed many of her strong Character Development moments and gave most of the rest to Aang.
At the beginning, Katara splashes an off-screen Sokka with water, but he is completely dry when he appears.
In what is purportedly an arctic area, you can't see anyone's breath.
At times, the Waterbending is not even animated.
The Firebending is pretty much nothing but poorly composed live-action fire elements with CG that even the film version of Spawn would reject.
The last-minute 3D conversion was universally panned for being barely noticeable and earned a one-off Golden Raspberry Award; Roger Ebert called it the nail in "low-rent 3D".
The first time Aang opens his hang glider from his staff, the support wires simply appear on the other side of his fingers. This makes even less sense than many of these, since the special effects department had no reason to cut corners like this — the whole of the hang glider was CGI.
Strangled by the Red String: Yue and Sokka's romance gets this treatment. In the show, their romance is developed and they get several scenes together. In this movie, it's reduced to exposition.
Surprisingly Improved Sequel: The spinoff comic for the movie, Zuko's Story, was actually regarded as a pretty decent read. Part of this is because its writers went out of their way to include references to the animated series.
They Changed It, Now It Sucks!: Long before the movie was anything more than a casting list and a few trailers, fans complained about changes to the source material. One of the biggest examples from the movie once it finally came out was the pronunciation changes, which Shyamalan deliberately included to make the character names sound closer to their "correct" Asian pronunciations (even though most of the names were made up) than the Americanized pronunciations from the cartoon. Fans were not pleased.
Shaun Toub and Dev Patel as General Iroh and Prince Zuko were noted to be amongst the better parts of the film for having genuinely emotional moments and for being faithful enough to the animated series.
Aasif Mandvi as Commander Zhao also puts in one of the movie's better performances.
The Agony Boothcalled out an apparent sexist tone in the film, pointing out that a number of important moments female characters had in the animated series (Katara's speech to the imprisoned Earthbenders, Yue realizing she can sacrifice herself to restore Waterbending) were given to male characters instead.
WTH, Casting Agency?: The casting of all the main characters was near-universally met with this reaction, with certain examples standing out:
Aasif Mandvi as Zhao. Mandvi is a primarily comedic actor best known for his work on The Daily Show. Shockingly enough though, he ended up being one of the most faithful characters to the source material.
Nicola Peltz and Jackson Rathbone as Katara and Sokka, given that most viewers of the cartoon viewed the blue-eyed, tan-skinned characters as Asian or Native American. Many years later, it came to light that Peltz received the part as a gesture to her billionaire father, which required Sokka to be cast with an actor who looked like her. Puzzlingly, the filmmakers went ahead and cast uniformly Asian and Inuit actors as Waterbender tribe extras, making the main characters look out of place in their own tribe.
Breather Level: "Surrounded" is so easy that it's almost funny. All you do is stand in the middle of the room and shoot at Fire Nation soldiers. You can get the level's collectables when you're not busy shooting fireballs, but that's it for that level.
That One Boss: The final Boss Battle with Zhao can be difficult at certain points. For example, closer to the final stage, he summons more soldiers to fight and he also gets down to fire at you. You'll get hit by Zhao's fire-blasting as well as the sword attacks.