Goat Simulator is an interesting "game." I put that in quotes because, really, it's a physics demo wrapped up in a ton of hilarious (an developer-approved) bugs and glitches. It's popularity is just about entirely viral, and not critical. It works because just the right elements came together. It's entertaining, don't get me wrong, but I doubt we'd be talking about this if the protagonist hadn't been a goat.
And now it's getting some sort of multiplayer, which will likely lead to some amusing new schenanigans. Or who knows, perhaps with the title gaining popularity, they'll suddenly go mainstream and do something more legit. Subvert expectations, an all.