It's the whole "interactive world" shit that really draws me in. I remember how stunned I was when you could blow up fire extinguishers in Duke Nukem! Things like Bethesda's open world games just make me hot. I want to run around collecting ferns and shit more than follow any sort of quest.
My ideal game is a totally open world space/planet thing. Fly around, go to deadville nowhere, discover a planet, land, get in Away Mission shenanigans, get back on the ship (or, not, build on the planet)...
But I'll take surviving a zombie apocalypse and running around anytime. Been flirting with the plethora of what's out there currently, but haven't yet been sold.