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 the 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 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 thought the protesters were not 'true' fans.
Firebending only being able to manipulate preexisting flames (except in special circumstances). People tend either to be upset that it's not the same as in the show, or feel that it now fits with the other three bending types in that it doesn't create the bendable element.
Dork Age: For Shyamalan, easily the lowest point of his, receiving universally bad reviews from professional critics and all corners of the Internet and receiving 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 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: In stark contrast to the characters they were based on, most of the cast was hit with this as audiences took issue with nearly every aspect of their performances from the acting all the way back to the actors chosen for the part. The poor writing for the film versions of the characters didn't help.
Francis Guinan as Pakku has been noted by several reviewers to be the one actor to escape the film with his dignity intact, putting 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.
Probably the ultimate example is the PebbleDance: As the Earthbender prisoners stage their revolt, six Earthbenders move in precise choreography, stomping the earth and releasing a mighty battle cry … and a tiny rock barely bigger than a football floats by very, very slowly. A seventh Earthbender then launches it at a guard. A fan theory suggested that the dancing Earthbenders were actually responsible for making the earth walls previously used to block the fire blasts, but the scene alludes to the cartoon, in which Haru's father was the one who made the earth wall. It probably would have been more effective if all seven benders instead each picked up a moderately sized rock and just threw them.
"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. In fact 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.
Internet Backdraft: Online criticism of the film reached apoplectic levels. Luckily, capital letters and boldface are unlimited resources.
Before the movie had even come out, early test screening reviews mentioned a scene that has turned the phrase "punching the fish" into a meme◊.note He actually stabs it with a knife.
"Bring me... all your elderly!" (Zuko's line when he's searching for the Avatar in the Southern Water Tribe makes it sound like he has a fetish for elderly people. He explains himself better in the show.)
The pebble dance, the most notorious example of Fight Scene Failure mentioned above.note Six Earthbenders. Four seconds of kata. One rock, slightly larger than a human head, gently floating along.
Kat-tackle (Katara pushing over a random guard and then just standing there).
Zhao constantly mentioning the secret library (Movie!Zhao mentions the library several times, as opposed to Animated!Zhao, who vaguely mentions it once).
Ozai's glowing ass (from a screenshot in which the source of light appears to be Ozai's rear end).
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 criticize 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.
The Scrappy: Katara's live-action incarnation has 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".
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.
They Changed It, Now It Sucks!: Long before the movie was anything more than a casting list and a few trailers, fans were complaining 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.
The Agony Boothcalls 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) are given to male characters instead.
WTH, Casting Agency?: The casting of all the main characters has been 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 the white actors looked nothing like the characters in the cartoon. This was made worse not just by the fact all the Waterbender extras were Asian or Inuit actors, meaning the main characters don't seem to belong to their own tribe, but also by their Dull Surprise acting.
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.