This actually reminds me of a game I played this past weekend at Otronicon. The game, which I think was an in-beta social simulation called OLive, wasn't particularly great, but the best part I had with the game was traveling to the very edge of the world map and falling off the bottom, only to hit the bottom of the room. I could do everything I could do normally down there--seeing as I never changed maps--, but the only thing I could see was a vast plain of sky blue ground; there wasn't even any indication that there was terrain above me when I looked up from a first person view.
Okay, so it's not exactly a minus world. But hell, I had fun seeing the game from a top-down perspective and watching my character disappear beneath the ground. Ahh, fun times. Oh, yeah, and the rest of the game sucked, too.