Killing Floor. The name itself inspired death, and destruction; and in most cases the game really lives up to its name.
Killing floor is a completely different type of zombie killing than L4D, so their really isn't any comparison to be made. KF is a wave based shooter, where you have 1 minute between each wave to go the trader (which rotates positions around the map every wave) and buy new weapons and armor. Not only are there a great variety of weapons and such, but you have a great 6-class based perk system, i.e. Commando. Berserker etc.
The perk system adds a interesting twist to the game as it allows you to customize your gameplay style, different types of weapons are specialized in each class, so each class is substantially different. This also adds a special type of addictiveness to the game, as you want to level up your character to the next level.
The Maps included in the game are really fun, and as long as you don't glitch in them, they can really make for some epic moments when playing online with friends. Talking about friends, this is great game to find new friends in, as the intensity of the combat in the game can really make you bond with someone else.
All in All, this game is great fun!
Pros:
-Fun gameplay
-Easy to use weapons
-Intense, bonding multiplayer
-New trader Aspect
-No high system reqs.
Cons:
-Small amount of maps
-Somewhat lackluster graphics