I disliked the movie, but it certainly isn’t for the reason, as @dreambender4 and @tierce put it, that they didn’t stay true to the characters or storyline. But it was because the movie’s acting and dialogue itself are poor.
Even though Aang was called ‘Ong’, either intentionally or unintentionally was no big matter. The reason being that M. Night Shyamalan wasn’t making an exact replica of Avatar: The Last Airbender cartoon. He merely took ideas from it, as it explicitly states, “based on Avatar: The Last Airbender”.
Yes, the executive producers were Michael Dante DiMartino and Bryan Konietzko, but they said in an interview that they had no real power during the film making process. They only aided M. Night Shyamalan at certain places and the rest were the directors own idea.
Throughout the movie, there were many clues of it being not an exact replica of ATLA, from them saying: “The fire nation doesn’t wish to live by the spirits. That’s why they’re so frightened by the Avatar.“, “As many fires in this city should be put out as possible when the alarm sounds. We want to minimise their bender sources.” to “Sozin’s comet is returning in three years.” and “It will give all firebenders the ability to use their own chi to create fire.”
As you can clearly see, M. Night Shyamalan felt that he could have made the storyline better by tweaking some key points of the ATLA world. Obviously judging from the majority of the fan’s reaction and the profit made from box office, he was wrong.
The acting may have been poor; true the dialogue was appalling. But was that their fault? No. They, like Mike and Bryan, had no power in the going-ons during filming. Personally, I think Noah Ringer did a great job. He was a bit shaky in the beginning, but I put that down to the dialogue he said. By the end, he was brilliant. He truly captured the spirit of an airbender.
Kudos also goes to James Newton Howard. One of the main reasons, and this is a big one, as to why I (even remotely) enjoyed The Last Airbender movie was because of the music. He used it to capture the emotion and flavour of the scenes brilliantly.
You have to remember that they are making a live action film, not an animated cartoon that’s 2 hours long. What would be the point of an animated cartoon that’s 2 hours long? It would just be like the Ember Island Players, but five times longer, or the entire three books of ATLA, but many times shorter.
In short, I am asking you to judge the film as an individual, not as a continuity to ATLA. M. Night Shyamalan is merely using the Avatar world and making up his own other designs. He is not making a film of ATLA.
On a final note, of which I am slightly disappointed in the film was (the other being me majorly disappointed in the dialogue):
Azula doesn’t have that air of confidence and pure evil. She’s like a little father’s pet…