L4D vs KF

L4D = Left 4 Dead, KF = Killing Floor
Both of them are zombie killing games which can be found in Steam.

L4D: the most irritating sound comes from the pipe bomb, which draws zombies toward it.
KF: the most irritating sound comes from the trader, which sounds like a mistress in front of a brothel (no offense but it really sounds like it to me) and draws all the players toward her. (try not to look at the portrait and you'll get what I mean)

L4D: Either you must travel around (campaign, versus) or you are stuck at a puny map which, even if you move around, you hardly call it traveling
KF: Travel or camp, your choice

L4D: Free guns at certain spots
KF: from the trader: "guns don't grow on trees"

L4D: You have characters of different age, gender, race, occupation... it's a harmonious society~
KF: All military men. Who cares? We are here to kick zombie asses!

L4D: There are special infected and normal horde
KF: All infected are special

L4D: You have bots for your teammate if you don't have 4 human players. Every game is like a movie (rerun).
KF: No bot teammates. No movies or stories. Only your team and the zombies on the stage.

L4D: You don't have to care what's behind, it's the past.
KF: You weld your own doors and make your own maze.

L4D: USD $44.99
KF: USD $14.99