I just now finished book 2, and I have to say that it was a far more balanced and well thought out season then book 1. I do have something I'm not quite clear on with the finale, and I have a theory I'd like to share.
So, Vaatu and Raava can never be killed. If one is destroyed, eventually it will be reborn from the other. I understand the symbolism, what with peace or chaos only able to exist alone for so long without creating the other. But Vaatu has been sealed for 10,000 years, meaning that he cannot fight or be destroyed, so the battle of light and darkness has essentially been postponed until indefinitely, up until he is released. Which he was, of course, but then he was 'destroyed' by Korra.
Now, since Unalaq was in his Dark Avatar state, we can forget about him. It's long been established that dying in the Avatar state will break the cycle of reincarnation, so barring some bullshit retcon or excuse making by the writers to get him back, Dark Avatar is finished forever. Vaatu, however, cannot be 'killed', so the question is, what happened to him?
My theory is that now the Avatar (and by this, I don't mean Korra, I mean all Avatars from here on) will be comprised of both Raava and Vaatu. Eventually, Vaatu WILL have to reform within Raava, which means that darkness and chaos will grow within the Avatar, where as before this, the Avatar has been a being of light. Now that the Avatar is no longer a bridge between the spiritual and material worlds, he/she would instead have to represent the balance required for a world of both material and spirit, thus having to hold light and dark in perfect harmony.
That's just how I've come to understand it, though. So far, most fan theories I've come across have been along the lines of "Nope, Vaatu dead, Avatar magic"