Why the final episode was a piece of bullshit? let's start in how taking bending from korra, but then creating a ex machina to gave her bending back is one of the worst things they could have done, not just that, but they gave korra the ability to gave bending backs, taking away the whole weight of action that Amon did. This just prove how little the writers were ready to make real sacrifices on the story, and how the lack of these sacrifices destroy the meaning of action from the character and obviously the empathy you feel. When Lin lost her bending was meaningful, not just because she was a character with a lot of spotlight, but she lost them for a good cause, now that she have her bending back, it make her action less of a sacrifice.
But you could argue "they didn't knew the avatar could gave bending", and I would just have to quote Amon as a kid who said "the avatar is the most powerful being" (not literal quote), which is true and why is so flawed to start a story with an almost full grown avatar. If Amon knew that the Avatar is unbeatable, then why would have he thought that his mimic on taking bending away would work? why he didn't thought the fact that if the avatar can take bending away, it can also gave it back? Or that if the avatar defeated his father, just entering the avatar state, what made him think that he could have won? The problem is that the whole story is around a flawed character who is the most OP being the world, against someone who basically did everything as a theater scene. The worst thing about this, is that Korra won with airbending, while Amon was still a full grown bloodbender.
Then we have Asami, Douchebag and Twat. How is possible that they have a emotional struggle and the best answer is "not now asami"? If everything, this is the best time to find what is going on. But okay, lets skip the horrible love triangle, and lets just say that the whole love story just teach us this, twats and douchebags are for each others, and any other character who was emotionally wounded mean nothing for us, making us, the equal douchebags and twats that these other 2 character are. If anything, Korra never was a strong female character, it was just a twat that only cared about her self, even in the end, everything was her "don't love me, because I can't bend anymore", what the deal with that? why not "don't love me, because we are just a bunch of assholes who wounded everyone"? The deal is, Korra, is the worst character ever, and the story was told in a way that, if we loved her, we would become as equally awful.
This critics can go forever, the whole episode 11 and 12 were just horrible.