I finally discovered Avatar a coupla weeks ago. It's probably the show I was least likely to find but most likely to like. I associate Nickelodeon with very kiddy fare, and reading the description of the show (which I'll spare you as it sounds cliched and simplistic), I almost passed it over. I'll never know why I didn't.
I just watched the last episode of the first season, and I'm not only impressed with the resolution, but its sustained level of quality; both in animation, but more importantly, in storytelling. The characters are each very well realized. Not just the main three, but characters on the opposite side, and even minor characters get some loving attention.
The jokes are genuinely witty, and even the touching moments are actually touching. Character designs are fitting, animals are cute, backgrounds are well drawn, clothing design is varied and detailed, the sound really give it that extra dimension without intruding, the voice acting is probably some of the most amazing I've heard in recent memory (I have a hard time remembering anything better). The dialogue runs from "nothing I'd complain about" to "who are these guys writing this stuff and how did they get roped into writing a cartoon?".
As far as design goes, the only black mark I'd give it is the ships, as they looked kind of meh. But not all mechanical design. For instance, some doors in the series seem to operate in a very realistic and plausible fashion. Of course, a show about amazing powers in a fantasy world would tend to invoke quite a suspension of disbelief, but I'd say their physics seem fairly consistent as well.
Some of the themes seemed rather advanced for children, though. I haven't been a child for quite some time, so I don't know the state of things, but I don't know how much of the spirituality and cultural aspects of the show they understand. Also, there were at least two different episodes that addressed gender inequality and sexism.
Speaking of culture, I really like all the Asian culture in the show. Of course, I don't think there are any direct analogues, but there's no denying some similarities. If I knew more about Asian history, I might be able to make some comparisons with the Romance of the Three Kingdoms of Warring states period, which I have some inkling of a clue that this might have something to do with. That said, I'll keep my mouth shut about which nation I think matches with which.
One specific gripe, which I think might have ramifications on the series later on concerns the battle at the end between the Fire nation and the Northern Water nation. It seems to me that the Fire nation should have pressed its reach advantage. Water could've countered with some specific attacks on their distanced weapons, such as with the Avatar or at night, when their powers were at their greatest. The other part of it is that since the Water nation's powers were greatest at night, it made no sense for them not to attack at night. They just sat their awaiting the Fire nation's next daylight attack. I think the writers could've had any number of attacks and ripostes, but that might be one of their failings. Or maybe the battle as it unfolded did have some sound strategical planning that I'm just missing. The fights between and among benders and warriors all seem to be pretty well choreographed, so I'll give them the benefit of a doubt for now.