Avatar: The Last Airbender is a phenomenal show that deeply develops its characters, tells its story in an action-packed, yet moving way, and makes us laugh along the way.
Character development: From the moment we meet each character, we get an idea of what kind of personality each one has. Over the course of its three seasons (called "books"), we watch as these characters grow and become real people (or about as real as element-bending cartoon people can be). The voice acting is superb, making these characters feel like more that drawings on paper. The animation is well-done, even when it gets into its cartoony moments (like someone foaming at the mouth from fanboying too much). Each character has a developing story arc that helps in deciding whether we like them or not. No character is too bland to the point we simply don't care.
The story: This is not a unique idea, but its execution is. Elemental powers has been done for ages, and this is no exception, but the idea of manipulating existing elements, how they draw their power, and the fact that only certain individuals can do it, while may not be unique, is pretty darn cool. The idea of a nation trying to conquer the world is not new either, but how it is done makes it feel fresher than it could have been. It does not take itself too serious, with plenty of humor scattered throughout, but its serious moments are not spoiled by the humor. The story is an absolute joy that will please all kinds of people of all ages.
I have watched this show twice all the way through and each time, it was sheer pleasure. I was sad when it ended after only three seasons, but I also felt it did not need more than that. I also feel the story needs nothing added to it. I understand The Legend of Korra is a thing, but I have no desire to watch it because I don't want it ruining the original. I may watch it someday, but since Avatar: The Last Airbender is practically perfect just the way it, that may never happen.