Only issue i had with Left 4 dead was just the cooperation forced down your throat.
In KF you have maybe 2 specimens that can instifrag you, and at least when that happens it happens because you made a mistake and get killed instantly. Thats the same for all of KF, you die if YOU make a mistake.
Left for Dead on the other hand has 4 infected out of 5 that can instant kill you when you are alone and L4D2 has 6 out of 8. But its not the instifragging that causes the problem, its the fact that you get pinned down and have to wait to die. I personally love fighting the tank or running from a witch, it is adrenaline pumping and fun. Getting pinned down by a Hunter, or grabbed by a Jockey, or tied up by a smoker isn't scary or exciting, its just annoying. Further more its cheap, "lets take all control off of them during the attack". I rarely die in Killing Floor and feel I was robbed. In Left 4 Dead I always feel like I just got slapped in the face by the game mechanics.
Obviously you should never go alone in either game, but sometimes your team gets killed. In KF you can at least have a go at surviving, L4D you may as well just jump of the nearest cliff. No matter how much life you are on, you can be 100% and it won't make a difference. 1 Hunter/Smoker/Jockey/Charger and your instantly dead.
I can't count the number of times I desperately limped to the safe room on 1 Hp, the rest of my team counting on me. I see a hunter , I punch it away then 1 shot it. A boomer makes a move, I pop it safely from a distance. I beat back a sudden wave of regular infected, I'm almost there, turn the corner into the safe room, turn round to press E.... and get grabbed out the door by a god damned Smoker from 50 Ft away. I rage quit.
So to sum up. Playing KF I have fun, playing L4D I rage.