So I rewatched LoK Season 2 this weekend because I needed some background noise while I worked and I was looking for something I'd already seen so I wouldn't be too distracted, and while watching I realised something: The season was actually very good. There were clear growth arcs, especially for Tenzin and Korra; a wonderful antihero in Varrick (who, honestly, is actually a greedy and terrible person that very efficiently masks that with humour) ; and the great humour and excellent animation intrinsic to the series. The villain was a little weak and his motivations a little unbelievable (as with all villains who actually want to destroy the world), but the rest of the cast made up for that.
It was the finale that ruined it for me and coloured my memory of the entire season, much like (but to a lesser extent) the finale of Season 1. Contrast with the classic build-up and foreshadowing that made Season 3's finale excellent and the overall freshness and awesomeness of Season 1, and it's easy to see why Season 2 gets the most hate in the series. I think seeing such an overuse of Deus Ex in that season after being burned by Season 1 is what got me and a lot of other fans, and led to us just dismissing the season entirely.
I know the show is officially done for good this time so such post facto analysis doesn't do much, but I guess it's good advice for all storytellers out there: Don't destroy a perfectly good story with a very shitty ending. If you end on a terrible note your audience probably won't remember what made your story great in the first place.