ATLA: the bending comes from watching animals bend.
Korra: it comes from fucking Lion Turtles.
ATLA: the avatar is the bridge between spirits and humans, so they cannot interfere with one another and vice-versa.
Korra: Fuck it! Let them be free.
ATLA: Thousands of past avatars to enrich the lore of the series.
Korra: whoopsie, i ereased them all, better start over.
Book 2 was a mistake, and I deeply think the writers acknowledge it,.
Besides that the only salvageable thing about Korra is the 3rd book.