L4D1/2 and Killing Floor are very different games, but for simplicity I'll draw it up to comparing features. 'Polish' will not be discussed as they both play well and are both aesthetically brilliant. However, since it's an opinion there will be bias:
-L4D has competitive multiplayer, whereas Killing Floor doesn't. It's very satisfying playing as the infected and introduces a whole new skillset as opposed to the shooty gameplay of the survivors. L4D supports fewer players in co-op, but there is 4v4 in versus. But then again I bought L4D for versus, not co-op. Both L4D2 and Killing Floor have mutations/mutators (new game modes with their own sets of rules).
-Gameplay in L4D is a lot more run-and-gun whereas Killing Floor requires you to position yourself tactically in approaching a horde or picking off the larger enemies in order not to have your innards gorged out by a chainsaw or a massive drill. There's also an emphasis on moving from A to B in L4D, whereas cooperatively Killing Floor requires you to set up...well, killing floors for the incoming horde. On single player however, you run more than you shoot in Killing Floor. It's not feasible to stop and shoot at everything you see right away.
L4D is more frantic in gunplay whereas Killing Floor requires some deliberation...that is until you get surrounded and all planning goes to hell.
-On the hardest difficulties (Expert Realism and Hell on Earth respectively) I would vouch for Killing Floor requiring more teamwork as the zombies are capable of sponging more damage. L4D requires more shooting skill, but Killing Floor places more emphasis on positioning yourself alongside your team. Sometimes you also have to known when to shoot and when to hold fire for the time being. There's also the aspect of managing your funds and sharing them with others. However, you do end up talking to your team more in L4D, especially if you're using Teamspeak or Ventrilo due to the high possibility of getting jumped by a player-immobilizing zombie like the Hunter or the Smoker from an unseen angle.
-Killing Floor has a wider selection and more importantly variety of guns. They also feel far meatier to shoot than L4D's toy guns. Both have melee weapons (and chainsaws!), but Killing Floor's manage to be far more satisfying as they pack more punch, giving you the feeling like you're actually ripping into flesh.
-L4D's stock and DLC maps capture their respective atmospheres very well, be it wandering through dark streets (for the first ten hours anyway, impressions wear thin with exposure), rushing up escalators in a mall or stuck in mud and rain during a thunderstorm. Killing Floor aims for a more stylistically deranged, hazardous and abandoned atmosphere. You can also see this in the character design and in the music. There are more official maps in their selection ranging from suburbs, a messed-up carnival to a misty forest to...there are too many to talk about in a single breath --> http://www.kf-wiki.com/wiki/List_of_official_maps
-Characters in L4D, despite all the hype don't really have all that much depth to them, much like the companion cube in Portal - but at least they stand out enough whereas the most you can do to stand out in Killing Floor is by choosing your avatar's appearance.
-Single player on L4D is pretty much the same as co-op online, but with the added aritificial stupidity of bots. The best experiences are found online with others and that's where you'll find your money's worth. Single player on Killing Floor has its own separate strategy and playstyle than in multiplayer and on those days when you don't feel like playing online, it can still be worth your money.