A diverse, politically-charged world. A sweeping story line. Complex characters with compelling motivations. Romance. Intrigue. Betrayal. Destiny. And here's the angst-ridden hero ... and his flying monkey.
Avatar: The Last Airbender is somewhat unique amongst cartoons because it gives kids some credit. Kids can understand convoluted story lines - their attention to minute details that only later become crucial plot points is astounding. They can handle ambiguous morality and divided loyalty. They can even appreciate the Buddhist principles that are the driving force of many of the hero's actions, while still enjoying the elaborately choreographed fight scenes that set Avatar apart. Kids are flexible - they can hold more than one thought in their head at a time.
Beyond that, the Avatar universe is appealing because it plays by its own rules. There are four elements: earth, water, fire, and air. Certain people are born with the ability to "bend" a particular element to their will. The Avatar is the only person in the world who can master all four elements, and restore balance to a world gone mad. Even with these fantastical concepts, there are no shortcuts or obvious plot holes in the series. And when the writers break with the traditional ideas of "bending," they do it in a way that stays true to the original concept. Deep down, all kids crave structure, and Avatar provides it in spades.
Not that the show takes itself too seriously, which is always important in children's programming. In my humble opinion, Avatar: The Last Airbender is one of the best cartoons on the market today. And with the closing of canon (in the form of a made-for-TV movie called Sozin's Comet: The Final Battle), now is the perfect time to sit down with a bucket of popcorn, the Avatar DVDs, and a precocious eight year-old to explain it all.