A great game, although a few things are still missing.
The game is set out that you are one of six British soldiers fighting against a horde of mutants. After a test in a biotechnological facility goes wrong, a virus is unleashed that springs up mutants that kill or infect any remaining humans. As one of six British soldiers (or just one if you are playing in solo mode) it is your job to fight all the infected you encounter until a trader is available for you to sell and purchase weapons and ammo.
As you can see the game is in a way similar to Left 4 Dead, but the fact that this was once an old mod means that it is not a rip-off in any way. It is difficult to review it without mentioning L4D, so first I will speak about it's better side. The game has a wide variety of weapons with each managing to feel different. These can be purchased by a trader who appears after every wave of mutants is destroyed. The game offers a very tactical advantage as you can seal doors which would divert or hold back your foes for a limited time. The game even offers you the option to play in third-person in singleplayer, which is a joy to watch when using a melee weapon.
The graphics are very dated, but considering that this was developed with the Unreal Engine 2, the physics and graphics look pretty darn good. Although not as gory as L4D2, it is much more violent than L4D1, and the ZED time feature which is a slow-mo feature that occurs from critical hits ends up giving you a good view of exploding heads and decapitation. As well as visual detail and good real-world physics, the game has a variety of maps to choose from, each which are non-linear. They include places like a farmhouse, a wrecked street, and even one of the labs. They are very well designed and a lot of them feature some gross secrets/aftermaths in some areas, committed by the infected.
The sound is not the best thing about this game, but it never feels too cheap. I have heard better sound effects for guns, but they are still satisfying enough to make the action enjoyable and intense (everything sounds best in slow-mo). The menu music is very atmospheric and fits the game well, although I cannot say the same for the in-game action music. Although the heavy metal songs are catchy, they make the action feel too stylised and not jumpy or frightening at all. At least, that is the way how I personally feel about the use of music. The voice-acting is also something that does not sound pleasing to the ear and could have been a lot better.
Excusing the bad stuff, these are the good points of the game. Gameplay-wise there is obviously more content than in the L4D games which is a great thing. However, the reasons why it still does not beat L4D2 at least, is because it misses some things. The game is lots of fun when you are doing multiplayer, but when you are in singleplayer you may get bored very quickly. Problem being that the game is repetitive. This is not an issue in some games, but here it seems to be. There are a few game modes, but they all usually involve nothing more than killing some monsters, grabbing stuff from trader (then repeat that process again and again). This would not be a problem if I was a multiplayer gamer, but because I spend more time on singleplayer this is not benefit for me and other singleplayer gamers.
Perhaps the reason why the repetitiveness was a huge effect for me was because I played L4D2 before trying this (L4D1's gameplay was quite repetitive) and after experiencing it's well-flowed, objective-based missions I was hoping for some of this here. Unfortunately, this was not so.
In short, Killing Floor is a very fun game with a very reasonable price of US$20 (though I was lucky to get 75% off). When I first saw it's gameplay, I thought it would be much cooler than L4D and would get a very high score from me. It has brilliant gameplay which will be very much enjoyed by multiplayer fanboys. The bad news is that the singleplayer's fun is the fun that lasts the shortest. Still, for this price this game is not bad at all.