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.
Eight Deadly Words: Unlike the animated series which thrived from its richly developed and complex cast, this film was massively panned for its inability to do the same with its interpretation of the show's beloved characters; fans and critics were in unanimous agreement that most of the actors came off as both awkward and boring, and too heavily detached to really engage with the viewers, thus very little of what made them so memorable on the show had shone through in the script.
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 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.
Fandom Berserk Button: For those fans of the source material who acknowledge it at all. Many Avatar fans refuse to admit that this movie even exists.
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.
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).
Yue's line about believing in beliefs, and her penis hair (how it looks like when viewed from the back◊.)
AANG! JUST ONCE! (Fans of the show got annoyed at how 'Aang' was pronounced 'Ahng.')
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 this script removed many of her strong Character Development moments and gave most of the rest to Aang.
Ship Tease: The ending of the Oasis fight was significantly more … Zutara-ish.
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.
Took the Bad Film Seriously: Shaun Toub and Dev Patel, though at least the latter was somewhat close to his character's original portrayal.
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.
Aasif Mandvi as Zhao. Mandvi is a primarily comedic actor best known for his work on The Daily Show. Shockingly enough though, he end up being one of the most faithful characters to the source material.
Many fans of the original cartoon would have preferred Asian actors to play Katara and Sokka because although the characters have ambiguous racial features, their culture is based on the Inuit. Caucasian actors Nicola Peltz and Jackson Rathbone were cast instead, but all the Waterbender extras were Asian or Inuit actors, so the main characters don't seem to belong to their own tribe.