Some games can be boiled down to one core component. Despite all that's been piled onto the series over the past two decades, for many people Street Fighter is about one thing and one thing only – Ken vs. Ryu. It's the one match-up that gets played ad infinitum, baring the very heart of the game and for thousands their foremost experience of Capcom's fighter in all its incarnations.
And in this way Survivor may well define Aliens vs. Predator. It's a mode that will keep people going long after the campaign's credits have rolled and the deathmatch numbers have dwindled.