Being a big fan of the original cartoon, I have to say that its design is much better suited for the long-run format. Also, if the movie isn't being done in a way that adults can appreciate, it's not being done right at all, since Avatar is one of the shows where the themes are pretty mature and the characters, as broadly cartoonish as they can be at times, are actually pretty nuanced and human. Which is a lot easier to manage if you're running twenty-odd twenty-minute episodes instead of a 1:45 movie.