I liked the fact that there were clear callbacks to the Earth and Fire finales from ATLA. Lots of people watch Korra for the nostalgia factor - that's one of the main reasons why the Avatar Wan arc was so well received, because it
felt like old Avatar (and also got rid of the Lion-turtle deus ex machina during Sozin's Comet). It didn't feel like they were "reusing" it, it felt like they wanted to draw clear parallels to ATLA - like this:
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source)
I personally loved the development, and I think it's been the best out of all the seasons so far. Mako has spent most of the season caught up in his old relationship with Korra (with some theories being that that's why he hasn't bent lightning before the finale; because his chi was blocked). His hug with Korra a few episodes back marks him "getting over it". Meanwhile we have Bolin becoming more and more capable in many areas, shown by his ability to lavabend along with his flowering relationship with Opal, so he's not just the clumsy comic relief anymore. Korra hasn't necessarily changed as much as a person, but that's what this next season is about, showing how she deals with being physically disabled (at least for a while) but also mentally crippled from both the hallucinations and Zaheer telling her that the world doesn't need her anymore.
The finale was also wonderfully poetic, one of my favorite things being that all four main members of the Red Lotus squad were done in by their own element, and more so, Zaheer's three companions being killed by their own bending. You had Lin and Suyin fighting as a team, and Tonraq / Jinora both fighting to protect Korra. Jinora receiving her tattoos at the end was not only one final nostalgia trip, but also clear symbolism of Jinora continuing Aang's legacy.
And sure, a lot of this stuff might seem too blunt or like it was presented too obviously, but you have to remember that this is a show on a children's network, despite the themes this season and in both shows, really, being much more mature than that.