Twitch!
There I was, Flak Cannon in hand and running around like a madman. Never stop moving! Always check your six! Watch your ammo! All those little rules you learn from playing a game for so long. Still, I died a lot. Still, I didnt care. The appeal of an FPS like the Unreal Tournaments and the Quakes has nothing to do with immersion in a story (though there is one), and has little to do with the graphics (though it doesnt hurt that they are top notch). It has everything to do with the game play dynamic. In the case of Unreal, as I re-discovered last night, I am physically tired after playing that game. The tension is phenomenal. With my roommate asleep in the other room it was hard, at times, to keep my exclamations quiet.
There are lots of ways to FPS, but the ones that make you twitch will always be top of my list.