they tried to focus on making Korra look like some badass instead of focusing on the role the Avatar, especially a new Avatar, would play in a more modern post-war world.
they took an interesting world, a place where there is political unrest and unease about inequalities, and made it just a means to Korra's end. any interesting allegory on social inequities or social ills were thrown away for some cheap twist. i mean, a whole political movement with people who were willing to commit terrorism disappeared overnight because "woah, we totally exposed him guys".
putting that aside, it didn't help that any significant plot was overshadowed by forced relationship and family drama. not even that it existed but that it didn't fit and didn't really contribute to the growth of the characters.
finally, every ending was lame or a deus ex machina.
Korra had a hell of a lot of potential but wasted nearly every opportunity to do something interesting or different for cheap and lazy action movie shit.