-Graphics
In my opinion if the game has horrible graphics and I've completed SP then it'll be hard for me to go back (i'm talking like extremely pixelated) If the game has good graphics then it gives you something to look at and go like "ZOMGosh tis is sooooo butifeul"

-Physics
I REALLY hate it when there is a death ANIMATION. Ugh. When a person is killed while they're in the air they look really stupid. I don't really care if the enviroment isn't destructible (but that would make me smile)

-Fun
I like when I can finish the game and want to go through it half-a-million more times (KotOR2)

-Variety
If there is no variety then the game will get boring really fast and you'll be saying to yourself "Y teh hek did i by dis?!" I like when gameplay is non-stop action regardless of how big the map is

-Multiplayer
Don't really care

i think the most important things are replayability/long play time, originality (no good americans vs. bad russians or bad commander/officer/whatever who is just using you for his plans which you find out in the second half of the game)

and of course atmoshere! or a good story, both in once would be even better but nowadays you can hardly hope for both in shooters

A good shooter does not need a storyline. Battlefield 2 has good graphics, gameplay, maps, and AI.
Battlefield 2 (A game that is awesome and has great mods) is a good example of a shooter. So is CoD.

Also good maps, textures and AI are important.