Doom is a fine milestone for an OS that intends to have graphical capabilities. Maybe Doom in text mode for other OSes :P

It's a known quantity, and doom has been ported to everything, so it shouldn't be too hard to make work either (he says, not having done it).

An OS that can run Doom is clearly capable of graphical interfaces with user interactivity, and maybe sound (although sound would be easy to leave out).