The characters are flatter than most pieces of carboard, the greenscreen is painfully obvious for a high budget film, the effects are lackluster, the 3D was a complete waste of time given that they hardly used it, they managed to include several plot holes despite having been given a complete, hole-less plot to begin with, they CAN'T PRONOUNCE ANYONE'S NAME (omg!), and their extras seem to have been picked up in a bargain bin with the result that they can't even keep each nation a consistent skin colour. I also find it highly suspect that the Fire Nation just happens to be (mostly) Middle Eastern, while all the other main characters are white (even if their nations aren't). Actually, the Fire Nation gives off a surprisingly decadent feel that reminds me very much of the stereotypes of Neroian Rome. Just, you know, in the Middle East.
It's very sad, but about the best thing I can say about the film is that the actor playing Sokka was kind of cute. And some of the sets were nice.
Bottom line? Go watch season one again. You'll enjoy it a whole lot more. And pray that they never make another one, because seriously. *shudder*
*totally going to bed now*