Ok, so first things first.
The ending sucks.
Don't get me bashed by lovers of this animated flick. I mean I love the ATLA flick. Mixing my genre interests in Martial Arts and Elements. Storywise, ATLA is great following story of Aang. But Korra? Hmmmm, really, people could throw in a couple of rotten tomatoes. Sorry creators, you just have got to ruin what you started at me. I just have to get this one out.
The First Book Air, shows a really interesting plot. But somehow, finishes the main suspense of what the hero wants to achieve. Or should I mean how the hero Master Airbending. Seriously!? She just somehow acquired bending at the end??? Maybe you could've just followed ATLA for how the story seems to be at one goal for the whole books.
2nd Book Spirits suggests a support of course from the other world. But To be honest, you could've given it yet again another way of doing things with it.
3rd book. Yeah, I came back to my idea. Why did you not think of expanding it to other form or Technique of bending for all or the heroes. Not just the Red Lotus.
4th Book. Oh! Come on! Yes Kuvira could've been one of the worst. You just have to make her a formidable foe.
In anyway, all of these are just thoughts I want to take out after all those years I decided just now to watch the flick. Really!
Just to leave a nice one on the last note: Thanks for showing me Wan. At least I enjoyed his era. Korra Sucks!