Karnage is one of the cleanest IO shooters you will find. The gameplay is smooth, the graphics are pleasing, and the weapon selection doesn't leave you wanting more. It is a big battle with many players, and there are many different modes to choose from if you get bored with one.
WASD to move
Mouse to aim and fire
Walk over ammo/shields/items to pick them up
Scroll wheel or Q/E to change weapons
Use the cover as best as possible, and that can also mean sticking behind teammates while they enter rooms first. Try to shoot grenades into rooms you are walking past, you might snag a lucky kill since the maps aren't very large.