I can't comment on co-op, but I can see how great that will be when the time comes.

Nothing I could add here that hasn't been said above, other than CrunchyBolt Nova's reference to Minecraft, hit's the nail on the head for me.

And yes, when you have put some time into exploring, you may feel it could do with more content, give it time, on the other hand that's any game of this genre, the journey and the atmosphere are more than worth the time and money spent.