Screw graphics... those are for whores. The Xbox's real contribution has been its on-board hard drive for things like saving multiple seasons of sports games with in-depth stats, never running out of save space, almost-instantaneous tracking and saving of every item/weapon/grenade in Halo's massive levels (for tricks like building your own weapon surplus or launching the Warthog), customized music soundtracks in some games, helping to enable new game concepts in titles like BLiNX, and for helping to enable the creative visions of future games like B.C. and Fable.
I have to admit enjoying my own guilty, superficial pleasure from the Xbox's superior sound in games - though, that could really be utilized better in the future for neglected game concepts like tracking enemies all around you by sound only, or new concepts in real-time music/rhythm games.