If you like killing zombies and decent map design then Killing Floor is a good choice.
Killing Floor is a multiplayer game at heart because you can join the fight with online players to form a team for up to 6 players. This is the strong point of this game as well as great map designs and a variety of weapons and perks.
Speaking of perks you can add perks such as classes in other game of this type. Commando, Berk, Ranged, Medic, etc. they are all there. As you progress through each wave you receive points and must travel to the trader to to refill your ammo or purchase new weapons. Speaking of weapons there are the basic pistols with a flash light, shot gun, machine guns, flame throwers, dual pistols which seem to be the best for close ranged combat and ranged weapons like sniper rifle and the lever action rifle.
The map design to me is excellent. You have basically five-six maps in solo - I believe. The Farm which you guessed it is a farm in the country at night that seems to be one of the harder maps since traders are spaced out farther and there is more room for a zombie ambush, Bio Lab that has tight corridors and small rooms but good vantage points for defense. The London map is OK but was my least favorite since it left a bad remix memory that leads to my negative experence with HellGate London a few years ago. The Manor is really cool because it contains different environments such as a cavern system and a spooky old mansion that is great to set up some last stands in. The Offices map is one of my favorite maps that brings you numerous floors and rooms to hide in and explore.
The zombies are cool but could use better animations. I believe they are stiff in some respects and a better variety of monsters would make this game more diverse. You have the basic upright two legged zombies - ones with a sword for a arm, fat Doom like zombies that explode green toxic acid when you shoot them in the stomach. You also have invisible zombies and scorpion looking creatures that are a pain to kill because they are low to the ground.
The game play consists of exploring different maps, finding traders after each zombie wave and joining with your friends to form teams of up to six in a multiplayer game. The graphics are similar to Red Orchestra Ostfront's as well as the textures and lighting. This game I believe uses the Unreal 3 engine. The sounds are fairly good as well but nothing worth noting.
I did find some things I disliked about this game. No matter where you are it seems zombies appear near you randomly even right on top of you which may be a some from of a bug. Usually this won't happen unless your on top of an object such as a car or crate.
Overall, Killing Floor is a good game for L4D and zombie fans alike. This is very similar to Trip Wire's other mods such as Red Orchestra Ostfront because they use similar textures and gameplay.