The finale wasn't that great, I didn't like how they turned Amon into an idiot who did some very visible waterbending in front of tons of people and I did not like that he used bloodbending to take bending away, I think that takes away a lot of the spiritual side of bending. What did he do exactly, stop the flow of the blood to the brain's bending center?
I would have preferred if they had given him some spiritual ability, they didn't have to use Koh or explain every little detail. He still could have been a waterbender (who they tricked into revealing his abilities publicly) but with some special abilities.
A spiritual reason would have also made more sense to explain how Korra could get her bending back quickly. I would have slightly reordered the events at the end, Korra should have unlocked the avatar state by herself, that could have reversed Amon's bending blocking and only after that would Aang have shown up to teach her energybending to restore the other victims' bending.
Aang just showing up in the last minute pretty much saying "You're now spiritual, here's the avatar state, your bending and energybending ... bye!" was bad writing and the show being planned as a mini series is not an excuse, it makes it worse! If you plan your entire story for 12 episodes it should be paced better! The final scene would have been acceptable if Nickelodeon had cut the episode order down to 12 and Bryan and Mike had to scramble to make a satisfying finale, but that wasn't the case.
There were a lot of good things about the finale, but Amon's sudden stupidity and Aang as an avatar ex machina were awful.