Can I paint and polyurethane my plyfloor or does it have to be covered?
Pro: cheap, and nothing prevents you from putting a real surface on if you find the look does not really appeal after a while.

Cons: Potential for nasty splinters; plywood splinters are miserable. Relatively loud (both to walk on, and acting as a reflector of sound - also loud to the level below if there is living space there).