I saw it last night, thought it was absolutely terrible. They purposefully changed the way they pronounce some of the names, and the result is that anyone who watched the show will cringe every single time they say Aang (So that it sounds like 'wong'), Saka (Soka), or Iro (E-ro, like saying the name of the letter). The plot will be entirely incomprehensible to those who haven't seen the first season multiple times, and they skip over a lot of important events.
Perhaps the most glaring change is the complete lack of comedy in the movie. The show is so awesome because of the comic relief woven into the story and characters, and it helps to develop some of the characters. The movie takes itself completely seriously, the acting is pretty terrible, and they surprise you at the end by letting you know they're going to change the timeline of the story in a huge, glaring way. I sincerely hope that they do not try to make a sequel to this movie.
There's a good reason that it has 8% on Rotten Tomato.