Wow. This show is awesome. I have seen all three seasons. There is not a single episode that isn't great.
It is a kids show on Nick, but there is a lot there that is not for the average kid.
I think it is very cool how the various airbending styles are based different martial art styles. I also like the pervasive philosophical overtones. Of course it is just a 30 minute show and they never get very deep, but it's more than most other shows.
The artwork is excellent, leaps and bounds beyond most animation. I actually have a collection of images form the show as my screen saver.
The characters are rich and complex and consistent. I love Uncle Iroh. In the episode "Tales from Ba Sing Se" his story is about his son's birthday, who was killed in battle trying to conquer Ba Sing Se. Heart breaking. I actually tear up when I see that one.
I like the humor, too. All in all might be my favorite TV show, definitely favorite animated show.
I'm kind of surprised nobody has posted here on this one.