I looked back at a post I put up in May with very few hours left in Lost. I'm trying to guess, very badly, what the altaworld will end up to be. However, I noticed this comment:
Sideways world can also be the afterlife, or purgatory, or whatever happens after the end of the world, where everyone has to try to work out their issues.
Well done, anonymous. Perhaps it wasn't "everyone," but you pretty much got it.