I'm going to have to agree to the people who've been saying the finale was a mixed bag. I still enjoyed it, but all the flaws that have been circling around the edges of the season so far really came to the fore in these last couple of episodes, and I think most of them would never have existed were it not for the fact that they've had to squeeze everything into 12 episodes. A few thoughts:
-The Makorra relationship still doesn't feel right to me at all - I wouldn't go as far as to say that they have no chemistry, as others have, but the "I love you"s came out of nowhere (they've known each other for weeks), and it seems that Asami's just been thrown by the wayside. That's gonna be awkward come season 2. Also, Bolin! He was on top form in the finale, but the Borra ship just kind of fizzled out and was never properly addressed. Much as I like him, he ought to be more than just comic relief.
-The juxtaposition of the airplane fight and Amon vs. Korra and Mako was pretty great on the whole, and was a nice callback to the multiple battles at the end of A:TLA, but there were a few too many deus ex machinas for my liking (Naga bounding in, Korra suddenly being able to airbend really well despite never doing it before, Amon suddenly not being able to defeat Makorra despite being such an obvious badass throughout the series), and Iroh could have done with some more characterisation other than being shown as a firebending badass, since he just sort of slips into Team Avatar without anyone saying anything about him (though his takedown of the planes was fucking awesome).
-Someone already commented on the unlikelihood of Amon suddenly using waterbending so rashly when he'd been hiding it for so long, but I was more irritated at Mako suddenly breaking Amon's bloodbending - if neither Toph nor Aang could do it in the Yakone flashbacks, I really don't think that Mako ought to be able to. Plus, Amon's motivations were never properly explained: did he really believe in equalising everyone, or was he just in it for power? Because if it was the former, why did he even hide his bending in the first place? He could have taken his own bending away and proven to everyone how sincere he was. Moreover, his supporters switched sides ridiculously quickly when they same him waterbending, which is disappointing because the writers spent a lot of time showing how many non-benders had legitimate grievances with bending. Lastly, I was glad when Amon escaped because he's a great antagonist and could just as easily be the main villain of season 2 or just pop in as a villain of the week type perhaps (see: Alpha in Dollhouse). His 'death' scene just felt like the writers trying to tie up all the loose ends (hopefully he isn't quite as dead as that explosion would have us believe). I can't see how any villain in season 2 could top him, unless it's some sort of angry spirit (Koh out for revenge, perhaps? The comic posted here earlier about Koh and Wan Shi Tong was amazing).
-Speaking of trying to tie up all the loose ends...I was really surprised that the writers decided to take away Korra's bending. Having her cope with the fallout from no longer being able to bend would provide an excellent character dynamic for season 2, and I'm sure the writers would have been able to provide her with a way of regaining her abilities, most probably by invoking the spirit world (which is almost certainly coming back in season 2 anyway). Instead, Aang just showed up and was all like 'lol jk, here's your bending back'. I know there's a limit to what sort of ending you can get away with on a kid's show, but this one just felt overly eager to wrap up everything.
I dunno, I sound very critical but it's only because I have very high standards when it comes to the Avatar series. I have enjoyed it very much, but a lot of intriguing elements/characters/ideas the series introduced simply haven't been developed as much as I would have liked or have been dropped completely.