Honestly, I don't even know why some people hate this series.
In my opinion I think this series is the definition of amazing, sure it has some flaws but that's what makes it great, it makes me realize that not everything in life is gonna go your way, and that you have to accept life the way it is, this is the first series that's made me feel like I could relate to it because the majority of it is kinda sad and honestly a lot of TV shows don't have that, that's what makes this show so unique.
(Good luck to the Guys Michael Dante DiMartino and Bryan Konietzko for their upcoming Avatar series, hope it's gonna be a blast :))