Shooters all come down to three things for me and BIA: EIB fulfills all three nicely:
1). A.I. -- I don't want to play straight up UT bots but I want the AI to be good enough on its own to maneuver through the levels, taking advantage of the environment, keeping dynamic so they're always gauging the situation as well as my movements, and more importantly, staying in character so that they feel lifelike in the context of the game. FEAR does a pretty good job in this respect, I think. The AI in FEAR isn't perfect, but it's challenging and keeps you company throughout your 10 hour walking tour of some water processing plant and a corporate office.
2). The weapons -- These are guns and as such, they need to feel heavy and lethal. The kickback, which is basically a combination of animation, sound, and control tweaks, has to feel believable and visceral. Again, look at FEAR.
3). Atmosphere -- 'nuff said. HL2, Condemned, FEAR, Doom 3, etc.