Sympathetic antagonists are one of my favorite things in fiction, if for no other reasons then that they add variety to the stories. If the enemy is nihilist or totalitarian evil then their degree of success is just a matter of how dark the pieces is. However, if the threat actually has a sympathetic cause, in this case standing up for ordinary people in a city largely run by those with special bending abilities, then it is far from clear how things will shake out.
I don't want to get into too much speculation at this point. Let me just say instead that if you were a fan of the first series be sure to check out Legend of Korra. This is particularly easy because the episodes are available, with commercials, for free online. Coming in without having seen the first series won't tell you as much about the world and will indirectly spoil some things about the ending. However, if you don't have an easy means to get a hold of it then I'd say that the core plotline should be perfectly enjoyable and the 70 year time skip means that the world in question is a very different one.