Yeah, I'd consider Book Two: Spirits to be fairly mediocre without Beginnings Part 1 and Part 2 too though I don't think it's fair to separate it. The rest of the season is pretty good after those episodes so it props up the terrible start yet leaves the season ultimately very uneven.
I was genuinely shocked when the Dark Avatar destroyed Raava. And I'm glad that the writers didn't completely cop out to that (like they did in the previous season with Korra restoring Lin's bending and such). Even though Korra ultimately saves/frees Raava, there were still real consequences to Raava being initially destroyed. That said, I liked the finale. I think it would have worked well functionally as a finale to the entire show (had it ended there) though it works particularly well in regards to the rest of the show. It acts as a soft reboot as the entire world changes and Korra no longer has any connection to her past lives. Aang constantly looked to Roku (and Kyoshi a bit) for guidance whereas Korra no longer had that resource. I think that added to what made Book Three: Change so good. As the Avatar, she's on her own. She's functionally the new Wan (while also undoing his decision to close the spirit portals), the new first Avatar. And while Wan felt like he failed by the end of his life, Korra didn't by the end of the show. I feel like she's more content, she had made the world a better, more peaceful place. So Book Two: Spirits when viewed alone is perhaps not great though I try to look it as the first act of a three act story.
Anyway, I'd be down for rewatching Avatar: The Last Airbender though I believe February 21 is around the time Netflix releases the third season of House of Cards so I suppose after that, I'd be down for rewatching and discussion.