For me, personally, I like my villains to have depth. Vaatu and Unalaq both had one-track minds with one, too-simple desire. Amon was amazing because while he had a simple goal, the topic was very complex. As a result, his methods ended up being morally wrong while the results could have been very right. It was a moral grey area that I found fascinating. It was ruined for me by the pettiness of the explanation for why he felt as he did. His dad abused him through forcing his brother and him to be perfect benders, so his solution was to get rid of bending. It was... boring, and as a result it was a huge letdown.
The Red Lotus, on the other hand, may have had a simple goal, but they, too, had a - in my mind - sympathetic cause. They simply went about doing it the wrong way. I mean... take Ba Sing Se, for instance. In a way, it's sort of like the United States' wage gap. You've got the 1% living big and easy at the top, controlling all of the power, and the little people get crapped on. How many people would love to overhaul that regime to set up something that actually cares about the citizens living in the country instead of just the rich looking out for the rich? I know I would. I don't want the people in charge to be assassinated to achieve it, though. I think the idea of people ruling themselves is actually really wonderful; I just realize that it's not possible due to the nature of humans. The Red Lotus didn't seem to get that, unfortunately, but that doesn't mean their ideals were wrong to have. It was really just their methods that made them bad guys (in much the same vein as Amon).
However, I did love that they were real people with real thoughts and real feelings. As little development time as they were given, I really feel like the writers did an okay job with all of them except Ming Hua. I actually found myself loving P'li and Ghazan, and while I didn't feel an emotional attachment to him as much as I did to P'li and Ghazan, I liked Zaheer as a character in all of his capabilities. While he and his group also did terrible things, I really love how dark the creators started making LoK with the Red Lotus, and I hope they continue on that path.
Once I moved in with my grandma, I got some huge doses of spiritual teachings and learned a lot about other cultures. One of the things that really drew me to A;TLA was the distinctly spiritual nature of it all. I think that's part of why I actually really enjoyed Zaheer and the Red Lotus.
Rtron, when it comes to Book 2, you really didn't miss much as far as the characters themselves go. They all kind of became caricatures of themselves which is why I feel like season two kind of sucked, but Book 3 really fixed all of that. The characters got a much-needed dose of development that they weren't able to really get in season 2, and after the first half where it was kind of just... fluffy and a little boring, the second half got dark and interesting and kind of awesome. For me, the last half of season three was so good that I consider it my favorite season even if the first half was just okay.