Killing floor is only a survival games, and i can say, the "tank" of killing floor has a minigun implanted at his arm, seriously, WTF is this? oh, a copy of nemesis of resident evil series
Now left 4 dead has way better gameplay, not the best graphics, but a nice gameplay.
left 4 dead is based on survival, but at the same time, adventure. in left 4 dead you can play in various maps, campaings, can play in versus, survival, and you can create your own campaings, maps, etc. etc.
see the differences now?