ok, stop comparing the two games, because both games focuses on two different things.
sure your thinking, 'they both kill zombies, whats the difference?'
although both games are coop, L4D demands more coop then any other game. it is build on going through 'stages', where you and 3 others try to escape alive. what makes L4D different from other games is that you can never, ever rambo. those who do, are the ones asking to get killed. yes easy and normal mode you can rambo a bit, but lets see you try when there is a hunter/jockey/smokers/charger/witch/tank on you.
long story short, if your team hates you, kiss your butt goodbye.
now, L4D offers many great stages and gameplay, but non can beat the versus mode. this i think is what makes L4D so successful. this mode pits you and 3 others against the infected, but those infected are controlled by other players. so now you are up against a enemy with a bit more brains in there step.
and i also like the feature that L4D2 has, which is the realism mode. hate those rambos on your team? well realism is where team work is required more then the regular mode. you cannot see the outline of your friends, which makes it more important to stick together. and shooting off zombies limbs take longer to kill which make headshots come in more use. this is the best of L4D2, but thats just me.
now Killing floor, well, lets put it this way, if you want to just go with some friends and kill zombies/monsters, well you dont need to look far. people have customized maps to fit 64 players, so you play with 63 other people in working together to kill the zombie horde. the game also features a variety of weapons and classes that specialize classes, from those who like to run and gun to those who like to run and chop, there is something for everyone.
its more simplified then L4D because, well, sometimes people want to just shoot damn zombies with friends instead of going from point to point or do missions. its just that simple. killing floor leaves you with that satisfy feeling that L4D some times dont give you. but you do have to work for that feeling. because unlike L4D, there is a final boss in killing floor. and it takes quite some team work and fire power to kill it. and many times, people dont come out alive from the encounter.
so to put it simply -
Left 4 Dead 2-
good long campaigns
good variety of weapons (lets face it, killing zombies with an axe, crowbar, and of course the bat, is just plain damn fun)
the best coop and versus modes you will ever find.
characters that you will just love.
many custom maps online (you have to look for them and install yourself)
if you love working together with others, then this is the game for you.
Killing Floor -
characters........... well there are, but they all have the same voices strangely. but its all the classes that counts, which you can choose from of six.
excellent variety of maps mostly the 64 player maps i think.
loads of guns to choose from.
team work is not that required, but works better with others.
if you want to just shoot something, then this is it.
so this is what i think about the two games. if you bought them over the sales from july 4th, you would have only spent $20 bucks and still have change from that to buy a few ice pops
but whichever you choose, hope it brings you some fun times and heres to hoping you can get both games soon.
also heres some vids.
killing floor
[ame]http://www.youtube.com/watch?v=MqBthptoSy8[/ame]
left 4 dead 2
[ame]http://www.youtube.com/watch?v=PewcieMXHSc[/ame]
i think when people ask for help on what they should buy rather a game or something else, people should not be bias. sorry if i sound stereotypical if my post has some bias comments on both games, but i tried my best to tell it from a reviewer point of view.