1.) It's paced lightning-fast compared to the rest of the series. I feel like to make the ending satisfying, it needed another episode or two to help maintain the pace. This oddly-fast pace kind of inflicts the other problems it has.
2.) You've heard the phrase "show, don't tell", right? Well, I feel like there was a lot of "tell" here, especially compared to the rest of the show and the one that came before it. The hobo community (btw, I DID like how they actually acknowledge the existence of the hobo from Episode 1 again) was said to be a place where benders and non-benders lived together in harmony. We never saw any of that. Ever. But the worst case of "tell" was the whole flashback of Tarrlok and Noatauk (sp?). Usually, when Avatar does flashbacks, there isn't a narrator; we see the flashback happening, we can infer things from it, and it's kind of powerful (think of TLA Episode "Zuko Alone", and you'll see what I mean). But in this episode, Tarrlok pretty much narrates the whole flashback, and it just feels like "Stop! Now that we've blown your mind, we have to take time out to explain." It's awkward and forced and jars with the whole show.
3.) While I don't mind the revelation that Amon is Tarrlok's brother and a Bloodbender, what Korra decides to do with that information is, frankly, idiotic. They decide to infiltrate an Equalist rally and yell what they've learned. I mean... it does sound a little ridiculous and possibly like something a Bender would say in a smear campaign against the Equalists. Couldn't they have thought of a more clever way to expose Amon? I'll admit that having the burn make-up to rebutt Korra's accusation was a clever move, but you know what would've been more genius (in my opinion)? Actually burning your face. Then the make-up wouldn't wash off.
4.) When and how did the Airbenders get captured? Especially Tenzin (my favorite character in this show, btw), who has shown to be one of the most competent Benders in Republic City? They flew away at the end of Episode 10. Lin tried her damnedest to wreck some airships so that they could get away. Lin also refused to tell Amon where they went at the risk of having her Earthbending sealed. Tenzin's been shown to be smarter than the rest of the council (except maybe Tarrlok, but that's debatable). How did he and his kids get caught? For that matter, how long has it been between Episodes 10 and 11, anyway? Maybe I'd be able to swallow this a bit better if we had some kind of timeframe. But as far as I can tell, this is a plothole so bad that Korra POINTS IT OUT, which is now the second time I can liken this finale to Shyamalan's disasterpiece (first being the relentless pace).
5.) Availability of Korra's Avatar talents. I usually ask "Where do I begin?" rhetorically while actually knowing how to start, but this time, this is such a mess that I can't figure out any other way to start on this besides chronologically. Amon / Noatauk actually succeeds in rendering Korra unable to bend water, earth, and fire. We don't really get an explanation as to how Amon seals bending (I'm guessing that he bends blood into a seal on their connection to the elements), so it's a little surprising that all three of those are gone at once. It's also a surprise that when she sees Mako about to fizzle out, she suddenly Airbends for the first time. How? Why? Why did it take this long? And why can she still do it when Amon sealed her bending? The best I can figure is that Amon never uses his Bloodbending to seal Airbending before, so he probably didn't get to learn how to do that. But the details of how he does it are still hazy, and so is that theory.
6.) Tarrlok and Amon. Are we supposed to feel sympathy for these guys? Are we supposed to feel sorry for them when Tarrlok blows up the boat? Seriously? Yes, they had tragic lives brought upon by their horrible father. That doesn't excuse the terrible things that they did.
7.) What in the bluish-gray Hell happened to Amon? He claimed to Korra during their first face-to-faceless meeting, "I have a plan, and I'm saving you for last." This got me to think that maybe he wouldn't just rob Benders of their abilities willy-nilly, that he had some method to his madness. Maybe he wanted to take down the Bending authority and show the world that it no longer needs Benders, or something like that. But no, he just wanted to take over the city. And then he lined Benders up in the night and sealed them. And he started to make big spectacles of how he was doing it. His motivation seems like something a whiny teenager would think when he's grounded: "My father's an ass! I hate the world!" And when he's found out, he runs away and begs his brother for forgiveness. What a heel-turn from cool villain to whiny pussy.
8.) Availability of Korra's Avatar powers, Part 2, or How Korra Got Her Groove Back. And THIS part bothered me even when I thought this ending was tolerable. Okay, so with Korra only able to Airbend, the team tries to get Katara to heal her. Katara fails, claiming she tried "everything she could". Are you sure, Katara? Maybe there's still something you could do that might be able to reverse Amon's damage. Something like BLOODBENDING?! Ya know, the thing that cut her connection to the elements in the first place?! And I would've actually been fine if the spirit of Aang had just showed up and talked to her briefly. It would've been cool if they just had the chat about hitting your lowest point or something and ended the show right then. But no, instead he Avatar Ex Machinas all of her powers back, making her better, also showing her how to do it to others, getting Tenzin to acknowledge her as "Avatar Korra", and...
9.) "What the fuck do we do now?" is what the creators are saying. The series ends with no real antagonists lingering about, virtually no mysteries that were developed (aside from "parents killed by Firebenders all around", maybe), and no real direction of what to do now. Bad guys are gone, Korra has her powers back, she can give other people their Bending back, all is right with the world. And while that might be preferable in reality, that's boring for TV. I do think that the creators have the potential to make something else interesting happen after this, but at the same time, this is the kind of set-up Disney sequels typically have to work with. The first one was meant to be self-contained, so any cash-ins, I mean follow-ups will be kind of yanked from out of the blue, and that could be damaging to the property's reputation. I'm waiting for what's next, just holding a much lower standard to it than before.
10.) Not as damaging to the ending as everything above, but Mako's still a douchebag.