Cast the Expert: Noah Ringer (Aang) was cast more for his martial arts experience (he got a black belt in Taekwondo at the age of 10) than his decent acting ability; in fact he had to take acting lessons before shooting started because he'd never acted in front of a greenscreen before. Ironically however he did seem to have the personality that would fit Aang (energetic, fun loving, etc).
Creator Killer: Shyamalan's career wasn'texactlyspotless before this film, but it looks to be the final nail in the coffin. For his next film, Devil, he was neither writer nor director and his name actually drew jeers from audiences when it appeared in trailers. His next directorial effort, After Earth, did not mention him in any promotional material and also avoided showing his name until the ending credits rolled.
Dawson Casting: Jackson Rathbone (24) as Sokka (15-16). Although that is the only example, as the others are played by actors of a closer age. Rathbone apparently had a good screen test with Peltz and very much looked like her older brother. He looks awkward playing opposite Dev Patel, who is six years younger despite playing a character one year his senior. Suki would like to have a word with you.
Disowned Adaptation: Avatar co-creators Michael DiMartino and Bryan Konietzko seem to share their fanbase's distaste for the film, having said in interviews that it distorted their vision of Avatar and have even advised people, including members of the original cast not to watch it.
From the same scene, Katara pushing one soldier to the ground and then just standing there doing nothing else is called the Ka-Tackle.
Franchise Killer: While the series itself lives on, the public response and critical failure of this film make any sequels unlikely at best.
Hilarious in Hindsight: Before its release, many fans were displeased that the release of James Cameron's Avatar had forced Nickelodeon and the producers to omit the word Avatar from the film's title (A complaint that resurfaced at the release of The Legend of Korra some time later); after seeing the final product, most fans would probably agree this wasn't such a bad thing after all.
Missing Trailer Scene: Two of them involve Sokka. One has him flirting with Yue by telling her "I taught him that," implying that he was talking about Aang. Another one had him cheering during the climactic battle after defeating the Fire Nation. It's pretty jarring that they were cut since they were pretty in-character for Sokka.
The first draft of the movie included all twenty episodes of the series, adding up to a whopping seven hours. Instead, the movie was cut down to barely two hours, and Paramount also had another half hour of film cut from theatrical release because they wanted to convert it into 3-D as quickly as possible.
Before things turned stillborn, Shyamalan planned to counter the 'Racebending' complaints in the sequel by casting Toph with an Asian actress.