I'm not sure, but I'd reckon a freeform world building sim, or something like that. It's definitely in a genre of its own, but I don't think the genre has been properly defined yet.
I've heard people call it an adventure game, but that doesn't do it any justice. Minecraft is so much more than an adventure game.
Is Uncharted 1 considered original?
The Path. Played it yesterday... or was it the day before?