This movie sucks. No, it's not because I'm an avatar purist who hates anything different from the show. It's just a bad movie. The main problem with this movie is that it tries to do too much in too little time. The original show had several episodes to develop characters, and the movie tries to cram too much of that into a movie. Some plot threads, like Sokka's relationship with Yue (something that plays a decently large part in the show), are included, but given so little attention that it would have been best to scrap them entirely. And character development, like Aang's water bending training or Sokka's character arc, is largely just ignored. When you try to cram the exposition, character arcs, plot twists and fights from a show into a single movie, it's nearly impossible to get something that works. And because the show spends half the time with (slow) action and the other half with expositional dialogue, it's kind of boring.
Also, I know I said this movie is not bad because it diverges from the show, but come on. I want something that's not just a live-action copy of the show, but they changed so much unnecessarily. I can live with the name changes, but the changes to bending and the characters' personalities? You have good source material. Don't ruin it.