Killing Floor has reigned supreme at every LAN party I have hosted two years in a row. By contrast L4D was played for nearly an hour before everyone got bored and booted up KF.
Killing Floor has the aspects of survival that L4D cannot offer.
Being a linear game, L4D seems less like you are being hunted, and more like you are hunting zombies. You have the option to play KF in such a way, but simultaneously you can be forced to run and hide. Nothing compares to the feeling of you and three other friends welding yourselves into a small room to hide while the horde menacingly pounds at the door.
L4D, by the aspect that you are in fact hunting the zombies, also lacks the fear aspect/ammo conservation aspect that Killing Floor has. So many situations result in there being 25 zombies remaining, and having only 20 shotgun shells to dispatch upon them. Running away in such a situation is the usually the best option, but the fact that you can only run so fast adds a terrifying aspect to the game. Constantly checking your six to see a horde of zombies tearing after you is great fun.
KF is also very arcade styled. It's simple to pick up and play. In the first five minutes of playing, everyone in my LAN group knew what to do, and we were already formulating strategies to conquer the maps. The control scheme is so simplified (meaning that you only really need about 5 keyboard buttons and a mouse to play), that even if you haven't touched an FPS in your life, you can sit down and play without a hassle.
Finally, KF is much slower than L4D in gameplay, by the very nature of movement speeds. This is not to say it is a slow game, rather your movement speed is a much more precious aspect. You can only run so fast, and picking up multiple weapons only hinders that limited speed. This is what leads me to my favorite aspect of the game:
KF is like a game of chess. It is a thinking man's game. Whereas L4D is technically run-and-gun, and all about reaction time, KF is all about thinking 10 steps ahead. Should you keep that grenade, or throw it? Should you shoot that zombie, or save your precious ammo and pull out your knife instead? If all of your friends are armed with slow firing weapons, perhaps you should pick up a rapid fire weapon to balance out your team. Every choice is critical to you and your team's survival.
Sorry for the long-windedness. It just seems that a lot of forumites affiliated with Tripwire's previous work is rather skeptical of KF's gameplay. Trust me when I say, "Give it a chance". It's an amazing game, and it's certainly worthy of your attention.