This review is going to touch both this episode and the previous one because both these two are some of the finest anime pieces I've ever seen. These last 6 episodes of this season have lifted season 3 to the point where it was just as good as the first season and this episode was easily the best episode of Avatar ever put to screen.
I'll touch some points in this review mainly with character development, villains and story.
Character: This show just showed through these 3 seasons how characters in an anime can develop and change for the good. Even Asami who was a pretty useless character in book 2 became such a devoted soldier that just reminded me of Juliet from LOST. Korra has developed tremendously from book 1. Not to say about the rest, Tenzin who I thought was going to die in episode 11 has become such a faithful friend, master and helper of Korra, then Mako and Bolin, especially Bolin who's story this season was that he could bend something more but he never could bend metal. And the fact at the end that he could bend lava was just how subverting expectations should be about (take lesson from that Star Wars!). Mako hasn't developed too much through book 3 as he did through book 2 but he had something to do just as important as anyone else, and by the way everyone played some role in this season.
Villain: I can honestly say that Zaheer was one of the best villains ever in anime and one of the best in television period! Hi s motives were so transparent and understandable but his ways were the ones that made him evil and worth rooting against. And the way he developed flying was extremely logic and I loved the fact that they teased this since the beginning of the season and the way it was executed in the penultimate episode was fantastic!
The story: And this is what makes LoK for me miles ahead of AtLA (say whatever you want) is that throughout this season there were pieces and hints that all came full circle at the end like Bolin metal bending and Zaheer flying. The story was so good planned and although it was a little slow at the beginning these last 6 episode elevated this season to heights that no other Avatar season or series ever reached and probably won't ever.
Just like some reviewers here I am going to say also that no other anime is going to match what has been done in this season season 1 and the second half of the second season. This show has reached some heights that I don't know if they're ever going to be reached yet alone surpassed. Absolutely fantastic! 10/10