Ouch.
I don't have friends, so that makes me 'noob' on their servers filled with co-op elite team plays.
I didn't like how L4D is really hard to play when your pals accidentally a witch (her whole being), or the fact at the very last minute, more than 2 Tanks can appear to block your escape route. To me, the very least of Killing Floor is a bit of a slight chance for the final stages to be won, rather than, OH GOD HELP ME, I AM PINNED. With the wider landscape and generally more open vision of the playing field with KF, I think it is easier to finish the last stages.
L4D zombies are too bloody fast, that is what scares me more. Solo play utterly impossible, one health pack/limited ammo grenades each stage that are normally hidden, not bought from a store = management, 4co-op is pure win.
KF zombies are generic, but really, because you can't shove them, and the fact they come in packs (Fleshpound fuck yeah) you are pretty much fucked, coming down to the servers and expect to play solo? No way dude, not in KF country. Also, you gotta admit invisible zombies is a cool idea, not as cool as a Witch or Tank, but pretty cool.
Also, point of note, FPS games are the only games I can play Hard and above without going wtfs.