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.