Well, being someone who just powered through seasons 2 and caught up in season 3... I feel like I want to jump in and voice my opinions about some stuff. 
First of all, I really don't get all the hate for season 2. I thought it, for the most part, was definitely better than season 1. Granted, I understand some of the complaints were basically that some stuff was rather frustrating just because you had to wait a week or two... only for an episode to end up mostly focusing on romance. So, I guess what I can say is that it's not that bad binge watching it. Even watching through season 3, I have to say that the whole story about the origin of the avatar is probably one of the best moments in Korra to date.
That being said... season 2 had a lot of missteps - primarily the ending. The "Unavaatu" goliath, Korra getting deus ex machina powers, Jinora doing... whatever it was that she did that didn't really seem necessary.
It started off silly, then just kept getting worse and worse. I feel like there were also a few plot holes. The most notable thing I have in my mind is going into the spirit world. Am I crazy, or wasn't it established in The Last Airbender that you shouldn't move a body when they go into the spirit world? I thought they said that if they were moved, the spirit would have a hard time getting back to the body? Also, they kept trying to go to the southern portal to break into the spirit world. Could they not have gone to the northern portal? It seems like it would have been guarded less, so it would have been easier to enter. And they had to have known that it was open because Unalaq needed it to be. (And if it wasn't, then there wouldn't have been any issue.)
Season 3 is ending up pretty good, though. The villains are interesting to me partially because they seem to have decent motives, and they aren't just the planning types like Amon and Unalaq. The romance seems to actually be subplots to the main plot, only taking up a small amount of the scenes. The animation has been great, and everything is moving along well. The show isn't dragging its feet at all it seems. Hopefully it can keep this momentum going!
So... good stuff overall. I feel like the first two seasons still get a bit too much hate, but season 3 is definitely blowing both out of the water. (Har har.)