What really makes Avatar great is that beneath the stuff for the younger audiences, some of the slapstick with Saka, or the other really childish jokes and bloodless violence, there's a much deeper side to the overall feeling of the story. It's a lot more mature than it appears. A good deal of the philosophical lessons and metaphysics as Aang learns about his powers comes directly from Hindu and Buddhist beliefs. After taking an Southeast Asian religion course in college, I was surprised how much I recognized buried in the series when I rewatched it.

Avatar is written very maturely, but also given plenty to appeal to the younger audiences that make up it's target demographic. It's like any really well written kid's movie. It's got everything it needs to keep the little ones happy, but it also is really enjoyable for adults.

Not to mention that each of the bending styles are adapted directly from real martial arts. They used a lot of reference animation from martial artists.

Fire - Shaolin
Water - Tai Chi
I forget what Earth was, and I was never able to recognize Air.