Really, aside from a lackluster B plot in Republic City (doesn't match the scope/urgency of the A plot, Asami and Bolin not believing Mako for weak reasons) Book 2 is pretty great. I don't know why everyone here has convinced themselves it's the bubonic plague.
Book 2 is, simply put, generally regarded as the biggest misstep or biggest fuck up of the series. Personally, I would argue that Book 4 is the worst season, but that's just me.
I think
Spirits is regarded as the worst for similar reasons that Season 4 of
Angel is considered the worst, although again I could make an argument that S3 is, but that's neither here nor there. My personal interpretation is not the nonsensical (yet fucking epic) ending, but that it's the character arcs that generally put the characters into a bad light. Hell, the season makes Aang seem like a terrible father, even though he really wasn't. Even so, I've made the argument that a character being unlikable can only be a bad or good thing. It basically boils down to whether or not them being unlikable is a logical progression of their character arc or the result of a "manufactured conflict," as I (and probably a lot of others) call it. I personally think the arcs in Book 2 are in fact logical, and thus they don't bother me.
Although, it does upset me that, out of the big "sins" that Book 2 commits, all of them are addressed or resolved in some form or fashion (although whether they're done well or not is a different matter). Spiritbending is never explained and greatly upsets me that this is the case. Seriously. How the fuck does spiritbending work? I mean, I can say what I think, but I want to KNOW.