Book 2 was just an awful mess. The first half was atrocious. Each episode had too much stuff crammed in. So there was no sense of pacing or time to actually feel what was going on. A lot of the characters regressed from Book 1 (completely ignoring their growth throughout that season). Plot/themes felt mostly incoherent.
Korra was a total asshat. While you can explain her behavior (and why she was acting the way she was), she was completely unpleasant. But really for me, I just think Book 2's writing was easily the worst in the entire series (like the actual pacing/structure of each episode). The only thing I'll give Book 2, is that the Harmonic Convergent plot was incredible (in concept). But even that, I felt was handled pretty poorly in the finale. Book 2 got its footing in the back half with Avatar Wan's episodes. But I really really hated the finale. Again, awesome concept, poorly executed.
When you think about it, the first half of Book 2 was a whole lot of nothing. You had a civil war plot that you didn't really care about. You had all the characters split off from each other, all doing their own thing. You had Korra running around being a terrible person. Every episode felt like a bunch of bullet point ideas (rather than organic plot) without ever connecting or coming together. And I really just felt that, unlike Book 1 and 3, they really were trying to balance way too many ideas.
Based on what I've read from interviews, it seems like production for Book 2 was hell.