what makes it better than any generic multiplayer shooter ?
The overkill.
Killing Floor is a coop, wave-based survival shooter. You and your teammates enter a map, pick a "class" (otherwise known as a perk) and fight off each wave. As you kill and complete each wave, you're awarded money. When the wave ends, you have a limited amount of time to reach the trader, buy new weapons, ammo and armor, then the next wave starts. If you can make it to the last wave, the Patriarch spawns, and you try to kill him. As you kill you also earn XP toward the perk you have currently equipped.
So what makes KF different than say, CoD Zombie mode? I'd say the intensity and the emphasis on teamwork. Guys spawn all around you, from multiple points. So you're rapidly surrounded if you don't have someone watching your back, or you have intimate knowledge of the level spawn points. The way to win KF is to find a good, defensible spot and hold fast. Later, tougher enemies require the whole team to focus on them and move a bit to not get splattered.
Unlike LFD, there's persistence to your character. Some people hate unlocks and ranks and stuff, personally, it gives me motivation to play. There will be 10 Perks (vs. the original 7), 25 levels with ~24 abilities to choose for your load out as you level up, everything from dealing more damage with your core weapons to showing enemy health bars to your whole team, welding doors faster, carrying more grenades, deep discounts on buying your class weaponry from trader so you can get sick guns earlier in the match, ect...
Also probably 6 players vs. L4D's 4.
Every player can weld doors shut to hold back the hordes and make an indefensible spot semi-defensible. Each player can heal themselves or others. The perk you're playing factors into this, making some perks healing specialists (always liked the Medic's MP5 with the medicated dart he could shoot other players with), others welding specialists (not my favorite to play), where others are straight damage with rifles or melee weapons or explosives.
Each map plays pretty differently depending on how camp spots and spawn points were designed. Lots of user-maps push these things to the extreme, making some maps way tougher than others.
Plus, it has great gore (which is supposedly even better in KF2.) Zed time, or slow time, activates when you do something cool like headshotting something. Some people find it annoying, but it never fails to make me cackle when a grenade goes off in a pack of clots and you watch them fly through the air in pieces in slow motion.
It's got a very "metal" design theme. So you know, heavy guitar, kind of in-your-face dialog. The whole game reaches b-movie levels of violence and gore for its own sake.
Special mention for the guns. TWI loves their guns and KF's guns felt great. Meaty, responsive, solid. Compared to L4D where shooting zombies felt like executing paper mache dolls, who would collapse and disappear before they'd even hit the ground, KF guns and enemies feel much more solid. You plug a dude with your 9mm and he reels. You unload a double barrel shotgun in their face at point blank and they go tumbling head over heels in rag doll. You cap them in the leg and they pitch forward on to their face. Unlike L4D, where you tend to see normal zombies as a horde and special infected as the actual, identifiable threat, each baddie in KF feels like a solid and real entity, so when you mow down an entire hallway of them.....it's very gratifying.
For $20 it was one of the best coop shooter games I've ever played. Put something like 6 or 700 hours into it. Eventually it does wear a little thin. You level up your perks, get the best guns and after a while waves stop being threatening when you know good spots to camp and the whole wave it walking into a wall of guns. But that's what the difficulty level is for.