1. Pronunciation. If you're going to make a movie based on a series where we KNOW how the character's name sounds, at least do it right.
2. Character quirks. We know from the animated series that both Aang and Sokka are the comic relief. I didn't get that vibe at all during the movie. Way too serious!
3. Appearance. When you see a live action portrayal, you should be able to make connections as soon as you see them. Zuko's scar is almost not noticeable at all, and Iroh is suppose to be a jolly fat wise man.
Too much time spent on CGI, not enough on thinking. This movie was probably designed by people who have never seen the animated series.