Avatar: The Last Airbender is a good show, but at times one can see that its target audience is composed of children - the show is too light-hearted, it tries too much to avoid grit.
Where is it written that adults can't appreciate a light-hearted, non-gritty story? Why should adults be trapped by such narrow definitions of what constitutes "adult" programming? Why should adults only watch programming that's forbidden to children, as opposed to programming that's accessible to children and adults alike?
The best children's shows have always had plenty of adult appeal --
Rocky and Bullwinkle, Doctor Who, Batman: TAS, Gargoyles, and so on. "Adult" programming doesn't have to mean only grit and violence and sex and overuse of four-letter Anglo-Saxon vocabulary. It should mean programming that's intelligent, imaginative, multifaceted, and well-made. And there's no reason in the world why children's programming can't fit those standards. Indeed, don't we owe it to our children to give them the very best we can offer? And if we do so, then the shows that result can be satisfying for children and adults alike.
And
Avatar: TLA was the best, smartest, richest children's show of the 2000s, and that makes it one of the best, smartest, richest
shows of the 2000s. It's more than good enough to satisfy any adult who defines adult sensibilities in terms of good, smart storytelling rather than R-rated content alone.