I don't think anything here was a deus ex machina. That refers to a contrived solution introduced without any setup or foundation in what's come before. I don't think that's the case. What we saw here was grounded in things that have been established going all the way back to A:TLA. Tenzin told Korra to do as the ancients did: rather than bending the elements, bend the energy within herself. That's almost a direct quote of what the Lion-Turtle said to Aang in "Sozin's Comet." The astral plane that Korra entered was the same one Aang accessed in "The Guru." And as I said, her giant astral form had a parallel in Aang's giant koi-spirit form in "The Siege of the North."
Yeah, they played fair for the most part...though perhaps that thing with Jinora might qualify. We still don't quite understand what the hell she did.
To be fair though, this may actually be set up for her story ar in book 3. We'll see.
That's why they were saying there'd be 10,000 years of darkness if Vaatu won: because it'd be that long before Raava could be reborn/reconstituted and challenge Vaatu once again. Instead, it turned out the other way around, and we'll have another 10,000 years of Raava and the Avatar in charge.
For me the interesting part is that the next time this happens, Vaatu will emerge from within the Avatar, once again bound to Raava.
I know the likelihood of them doing anything with that concept is slim, but I can't help wonder what would happen to an Avatar with both those cosmic spirits within them...also, what the world would look like after another 10,000 years. Would living with the spirits make humans regress, or would they continue to develop their technology?
I suddenly have a mental image of a universe that looks like 'Dune' crossed with 'Final Fantasy: The Spirits Within' and 'Bausicaa of the Valley of the Wind'.