Well, it's hard to tell with anime styling, but Katara and Sokka have some Inuit features, and the Fire Nation (and to a lesser extent Earth Nation) is obviously designed to be a riff off of China. Really, I think that only the signature character Ang is obviously Caucasion.

But this is really more of a "business as usual" sort of thing for Hollywood. Just wait until they get around to doing a Che Guevara film- it'll be Keanu all the way.