Haha, well I am optimistic. I can imagine it working well under various constraints, e.g.: a user joins, a new room is generated and attached to a dungeon, the user can only add things within the bounds of that space. An auto-moderator tracks the changes and only applies them globally if the rules are followed. Easy to grief but so be it. ;)
Another idea: everyone builds their own self-contained worlds (based on some common template) and a global portal world connects them up. A voting system shows a score above the portals so you know what you're getting into when you walk through a door.