Korra is obviously the star of the show, and her character (re)growth over the past season really shows here. She doesn't scream at people when she doesn't get her way, and she handles her relationships with others very maturely. At the end, she's decisive in her final actions and choices, but still listens to the advice of Tenzin and her friends. It's nice to see her progress so far from the brat she was at the beginning of this season.
Bolin also gets a lot of the spotlight this time around. We get to see that despite his earlier desire to get out of his brother's shadow, he still cares for him, and visits him in prison to inform him he'll cover the legal matters. All with typical Bolin humor. Aside from that, he gets to strut his bending stuff while saving the President himself in (appropriately) the pro-bending arena. He also manages to contribute to saving the world by basically sacrificing himself to Eska (that takes guts) and going toe to toe with Unalaq. He's even thoughtful enough to give his brother a hug when Mako really needs it.
Mako actually gets some characterization this time around. He gets validated in Lin's eyes when Varick's plot is revealed, and resists blowing up on everyone for not believing him in the first place. Furthermore, he shows that he's genuinely uncomfortable with trying to tell Korra what exactly happened between them before she left, aka the breakup (I knew amnesia would be used in some contrived way). In the end, he manages to handle it wisely and maturely and doesn't try to prop his pride when everything comes down.
Asami gets a little shafted in regards to screentime. It makes sense as she's a nonbender in a playing field made for benders and spiritual heavyweights. However, she does get her moments in the limelight when flying that plane in strafing runs, and she looks pretty damn pissed when Korra makes out with Mako right in front of her. More on that later though.
Tenzin, Kya, and Bumi get some nice sibling dynamics going on throughout the finale. Tenzin gets the most focus as he tries to save Jinora and help Korra spiritually. In the end, this means confronting his own insecurities regarding his father's legacy and accepting that he can't teach Korra absolutely everything. Kya manages to save everyone with spirit water a few times, but other than that, she mostly just plays off her brothers. Bumi meanwhile, gets to utterly wreck a Northern Water tribe military base purely by accident, saving everyone in the process. When he's not being an accidental badass, he's lightening the mood with some hilarious quips.
Eska and Desna get a bit more of a role this time, mainly playing the co-dragons to their father's evil plan, while at the same time doubting his motive and methods. They're still a little flat, but manage to provide some offbeat humor with their deadpan seriousness. In the end, they make choices like rational people (in Desna's case) or obsessive stalker people (in Eska's case) and defect for entirely valid reasons. Yea, your dad becoming the avatar of evil itself might be a little blow to filial piety.
Finally, Varick manages to still be pure gold whenever he's on screen, even though he effectively disappears from the plot after the second episode. Whether it's revealing that he had a special luxury cell commissioned at the jail because he felt that he might be arrested someday, or naming a battleship after his assistant, or escaping via flying machine after the prison is attacked, he's always just hilarious to watch.
Alright, enough about characters. The actual ending is here:
After a terrific giant fight, Korra manages to get rid of Unalaq and Vatu rather efficiently with some help from Jinora. When they get back to fusing after Rava almost dies, it's revealed that Korra can no longer contact past avatars. It isn't clear if no more avatars past her can ever do this or it's just her, or if this only effects her. However, to go with the change in spiritual scenery, Korra decides to try something new and keep the portals open to let spirits into the physical world again. It should be interesting to see how the humans cope with this new status quo in the near future. Definitely plot points for the next season.
Also, a couple ships apparently got sunk this episode. Eska x Bolin realized it wouldn't work out even though Bolin confesses that he likes Eska anyways. Eska and Desna are apparently heading back home to take care of their mother, and Bolin is heading back to Republic City himself. Neither of them want a long distance relationship, so they just decided to end it here. Similarly, Korra x Mako got broken up again, for real this time. After Korra recovers all he memories, Mako admits that their fight really was that bad and he broke up with her. Korra then reveals that she already knows and both of them admit that their relationship won't work. After one last kiss, they break up officially. Looks like this is the end of the love triangle (hopefully...).