There are loads of things that I hate. First of all, they cut out loads of scenes that were supposed to be vital, like Katara wanting a master to teach her water bending. They also made Sokka be so serious and that leader type personality even though he lacked both, and he was still awesome in the series. I didn't like that they pronounced some names wrong. I mean, really? Is it that hard to say Aang's name right. They said Aang as Ong and Sokka as Sowka. I acknowledge that in movie adaptations, they will change some elements of the original show for the movie, but they overdid it. Also, Aang was quirky in the show but movie "Ong" lacked such that made Aang Aang. I remember that Appa was a great side character even though he isn't human, but in the movie, he is just there. I mean, he doesn't have a role. If I was the writer or in charge of the movie, I would split it into multiple movie considering how much episodes and the duration of each one. I didn't like it.