I think the ending was drawn too far from the original spirit of the series. Half of the whole second season consists of Aang dealing with his feelings about the way he acted in his Avatar state in the northern water tribe. It will change a lot for the second movie, but it also provides a way to cut out all that long, dawn-out coming to terms with his power by having him skip it altogether (saves time in movie number 2) I also didn't like the mispronunciation of a lot of the names. I wish the classic avatar humor had not been cut out of the film altogether, and I would like to have had more time spent on Zuko and (especially) Iroh.
That being said, the cinematography and music were beautiful. The stunts were well-coordinated, and the emotions were portrayed powerfully - from Zuko's frustration and embarrassment to Aang's childish rage, Sokka's bravado, to Katara's invincible hope. The climax was very moving and the ending is inspiring. Not all of Shyamalan's best qualities shine through, but it was a good movie overall.