I was on walterman's group yesterday, initial impresion is that it sucks. Average graphics and below average animations. You can't compare it with Left 4 Dead, Valve title is miles ahead by far. My impression is that Killing Floor is much more a survival title, it's really easy to run out of ammo and you have to start using knife.
Maps are huge, you can go wherever you want. Game sends waves of zombies and you have a counter of how many are alive. Once all are dead you can go to a shop and you have 1 minute to buy ammo, weapons and upgrades before the next wave arrives.
It's like Invasion for Unreal Tournament 2004, but massive. Most people on the group thought that game really sucked, however I think it can be fun and addictive, but gameplay is radically different from Left 4 Dead, you don't have unlimited pistol ammo and every shot counts.
After a couple of matches in Left 4 Dead you know what to do, but not in Killing Floor, the huge maps and the total freedom can be a problem if you are used to run from saferoom to saferoom.
I want to keep playing and learn, but I'm not sure on how many people on our group will want to do the same. Looking at this
Steam thread asking about opinions on the game, it looks like almost everybody is enjoying it, even people hated it at the beginning, and started loving it after a couple of matches.