yay my turn to curb stomp L4D, im going to start by confessing that i dont own or have ever played L4D, on the other hand ive only played like 2-3 games of the killing floor mod (due to ****ing cd key error when i installed mega pack to fix missing textures, but cause its the steam version it got patched with a retail version patch n pissed itself and died started *****ing about CD keys...i never re-instaled KF again, but it was a mighty fine game if very poorly presented). anyway:
killing floor is gritty and atmospeheric, has more weapons, wider array of "mutant"'s, an SDK meaning instant mods/maps (plus UT editor is a cake walk compared to source >.>), persistent perks, open plan maps, srategy (not just A-B + kill zombies inbetween), and its got acheievments and stats just like L4D, has server browser, and is defo gona be cheaper ^^, only thing L4D has in its favour is shiny graphics, AI director, and shocks,