Killing Floor shouldn't be a great game. It shows its origins as a fan-made 'mod' in so many ways, like only having a couple of voice actors for everyone in the whole game, and some irritating typos and bad grammar. (A character called Captian Wiggins?!)
The trouble is, it's just so very addictive. As you play, actions such as headshots, healing team mates or welding doors earn you progress towards character improvements in seven different classes, called 'perks'. Want to carry more stuff? There's a perk for that. Want to get cheaper explosives? Ditto. So the more you play, the better it gets. High level perks spawn with free equipment, which is also nice. You don't really play Killing Floor to kill zombies, but to rack up experience to advance your perks.
As for a storyline, it's virtually absent: a marked contrast to the linear nature of that other zombie apocalypse game, Left 4 Dead. Killing Floor tries to tell a story, about how Horzine Industries unleashed a horde of clones on London... but the maps are played in no particular order, and each simply gives a paragraph in iffy English about the situation you face. Sometimes the trouble you face is described as an 'infection', sometimes not. But you know what? Forget about the backstory! This isn't Left 4 Dead, with its cinematic cutscenes and sense of progress towards getting out of the zombie-infested area... this is a deliberate mission to kill everything that moves, London style. So what if every mission always involves killing the same number of monsters in the same number of waves. 'Space Invaders' was successful...
True to its fan-made roots, Killing Floor has expanded a great deal since Tripwire Interactive launched it in 2009. It originally had just five maps, but several official expansions have taken place since. Each adds new achievements; particularly the special Christmas-themed one that was released in late 2010. Other improvements have included adding a new, higher difficulty level, and a seventh perk. The 'live' nature of this game and community means it can keep on surprising players for a long time to come - although we have to hope that a planned sequel doesn't bring Killing Floor to an untimely end.
Killing Floor is the processed cheese of computer games: a guilty pleasure - but a pleasure all the same.