My biggest quarrel with Legend of Korra is that Mako bent lightning in the first season even though The Last Airbender made a huge deal out it and explained that lightning bending was the epitomy of firebending and only masters of the art would be able to do it. Iroh couldn't bend lightning without huge ceremonial gestures, and Zuko was only able to redirect lightning. Yet Mako, who grew up on the streets and taught himself bending, was able to bend lightning like it was nobody's business. Actually, the place where Mako worked part-time in Season 1 had a ton of lightning benders charging generators.
Granted, Mako hasn't been bending lightning in season 2 or 3 at all, so maybe the writers realized how much they screwed up ^^"
Another thing is the completely unnecessary killing of the antagonists at the end of season 1. Would it have been too much to let them sail to a remote water tribe village and live the rest of their lives in peace? And how does a simple gasoline explosion kill someone who effortlessly battled countless benders?
Aside from that, I like Korra for what it is. It's good entertainment and season 3 has some good character developement so far in my opinion.