The problems with Korra are like 99% Nickelodeon meddling and fucking shit up. I've said it a thousand times, but here's the breakdown:
Season 1 was the ENTIRE SHOW at first, hence why it is entirely self-contained and doesn't leave room for continuation.
Season 2 ripped open the ending of the first season, it was rushed and it shows. A few good ideas that were probably cut early in season 1 managed to get through, like the whole story of Wan.
Season 3 was where the show started to recover from the meddling and get on a good track again. Honestly I think Zaheer and his companions were amazing antagonists and they should have been introduced in season 2.
Season 4 was, in my opinion, fucking phenomenal. Zelda Williams' character (whose name I can never remember because I get stuck thinking of her as Zelda Williams) was possibly the most unique antagonist of the series, and really drove home the "World War I" era feeling.
The issues with the show mostly lie in the transition from season 1 to 2, the fact that the show was never meant to be more than one season, and that everything after season 1 just felt incomplete and/or rushed, like they didn't have time to really execute all their ideas. The fact that the series is any good at all is a testament to the writers, but it could have and should have been better.