What I think most people like about the worldbuilding is the rich believable cultures that manage to be very obviously inspired by real-world Asian cultures, without being either caricatures or carbon copies. That it has a magic system that is woven seamlessly through the society of the world and doesn't at all feel like "a medieval society with magic added on as an afterthought". That the magic system itself does a great job of providing examples for all three of Sanderson's Laws, being a robust hard magic system with very obvious limitations that gets explored in depth through things like ice and blood and lightning as believable extensions of what already exists. Revelations that make you go "oh yeah, of course!" and "holy shit that's cool!" simultaneously.

In terms of characters, Aang's certainly not the most interesting. I think it's a bit more interesting than you give it credit for, because it's not just an "unwilling messiah", but it's a literal child who's forced to be a saviour. Not the teenager or young adult that you'd get with most YA fiction, but someone very much characterised as a young child forced to grow up too early. Themes of loss of innocence and the tragedy of war get explored well through him. But as I said, he's not really the focus.

Zuko is. Zuko's character arc is frequently cited as one of the best redemption arcs in fiction, and for good reason. It's one of the best written, most believable arcs I've seen. He starts out a bad guy, because he was born on the side of the bad guys. But we later learn that he was banished because he stood up for the little guy. And even though banished, he doesn't suddenly change the allegiances he's grown up with his whole life. He believes he can restore his honour by aiding the bad guys and taking out the main good guy. Of course it's actually a rather classic case of "he needed to find acceptance from within and stop seeking external validation", which is a well-trod trope, but it's his path to get there that's great. Thanks to his contact with the good guys, and the helpful but non-pushy guidance of his uncle, he slowly comes to appreciate that he can be better. But it's rather shallow and focused on just himself, not on understanding that the ruler of the bad guys is a bad guy. And so when the ruler welcomes him back, he goes. It's not a smooth curve from bad guy to good guy, there's a very prominent relapse along the way, due in part to the misaligned reason for his initial growth.

Other characters are also great. Azula's tragic descent into madness, shown as an extremely believable progression resulting from traits she exhibited in her very first scenes. The understanding that Ozai might be irredeemable because he's been steeped in the evil of the Fire Nation his whole life, but that the war didn't start out like that: Sozin was ego-driven and insane, but ultimately got there because of his tragic friendship with Roku turning sour after he was just spreading his nation's prosperity. Not a redeeming quality by any means, but an understandable one. There's Hama; captured and tortured for years until she got the chance to turn the tables and became the torturer. And of course, Iroh. So much could be said about Iroh.

Of course there are other things outside of that. The music is sublime. The representation is great—I've already mentioned the fact that it's focused on Asian and indigenous cultures, but also for people with a variety of different disabilities. The fact that it balances tone incredibly well with its serious moments and the light-hearted humour. The fact that it is, without a doubt, a children's show that nonetheless does not condescend to kids, and even manages to have depth enough to attract a lot of adults (and know personally multiple, as well as having seen stories of people online, who were first introduced to it as adults, so it's not just nostalgia).

I just...yeah. I cannot say enough good things about it.