I caught a lot of flack for criticizing it before, now after watching the season finale of Book Spirits on Nick.com I can now say Legend of Korra lives up to its predecessor.
Korra is a fully realized Avatar and has actually matured in this season. Retouching on things hinted at vaguely in Book Earth, showing the true power of the Avatar spirit, Cosmic Energy.
How she won the last battle isn't really described though (Vaatu was disintegrated but we learn that he and Raava cannot be destroyed). I would WMG that it's supposed to be vague enough for Vaatu to return again in some form.
They leave a lot of possibilities for the next story by opening the portal between the human and spirit worlds. Perhaps now she can talk to even the "disconnected" spirits on their own terms (I won't spoil too much).
Sorry Makorra fans, but this was hinted at when we found out about Lin and Tenzin breaking up. This actually adds a dimension of realism to the show as well. Aang and Katara were pretty much a fluke, in that they stayed together since their teen/preteen years.
(Bolin X Korra 4 ever!!!!!!!!!!)
If you're impatient like me, watch it on Nick.com before they remove it. Otherwise, wait until Friday.