Having put 400 or so hours into Killing Floor, here's my take.
It and L4D have little in common.
L4D is a game where standing still means you die.
Killing Floor is a game in which moving means you die.
Basically, Killing Floor enemies have more health and can do way, WAY more damage to a player than L4D's cardboard cutout zombies. KF enemies substitute numbers for lethality, and surprise rushes of guys for constant streams of reinforcements. This is offset by the weapons of Killing Floor players scaling where as L4D weapons don't.
In L4D, the way you win is you run, really fast, toward the objective and try not to get bogged down too often. The more you stop, the more zombies come after you, the more health you lose and everyone eventually gets incapped.
In Killing Floor, the only way to survive each wave is to reduce the # of attack avenues to two, or three at worst, where you can see and shoot the enemy long before they can reach you. The worst defense spot is someplace with multiple avenues of attack, where things can be hidden from view until they're almost right on top of you.
Killing Floor is designed in such a way that guys are coming at you constantly, from as many angles as possible, so you need the massed firepower of your whole team to kill them all. You're generally far, far less effective at short to point blank range, because by the time you've killed something, its partner is right behind it and in your face. You don't mow down waves of guys like in L4D, rather you smash holes through their ranks.
There's tons of variety in the enemy types. Some are designed to hold you in place, like Clots. Some are designed to nip at your heels and leach your health, like the Crawlers. Some are designed to inflict serious pain on grouped up survivors, like the Husk. Some are designed to attack THROUGH other enemies, like the Siren. And some are designed to straight bum rush your group and turn you into meat chunks, like the Gorefast, Skrake and Flesh Pound. The point is, all the design elements of KF come together such that the best way to live is to just keep everything as far away from you as possible, coming from one direction so you never get blindsided by something evil.
And guess what? The worst place to be is backed into a corner! Because despite what I said, your group does have to be able to move forward and backward a bit as the waves intensify. So forget finding a long corridor with only 1 entrance. Besides, that corridor probably has a ventilation shaft or something that enemies come out of, conveniently placed IN YO GRILLE.
Ideal defense spots are very level-specific. It may not seem so at first, one place might seem as good as another. But as the difficulty increases, every defense spot starts showing its weaknesses. Even good, clear lanes of fire can start becoming a liability in later waves when you've got two or thee husks taking pot shots at you. KF is just one of those games where it all comes together so nothing is every truly defensible or ironclad.
So, the basic rules are these:
-Go where your team goes. Almost no one survives a wave of KF alone. This is the BIGGEST thing that people who play L4D don't understand about KF. KF is not a one-man show, there's simply too much out there, coming too fast, from too many directions, with too many special abilities, for one person to deal with. Those that do survive solo are ones who are set up to kite, and it can take 10 minutes for them to kite an entire wave by themselves. It's also highly dependent on the level whether they can even do this, and involves manipulating the spawn mechanic so they can get everything following them from one side. When you're new to the game, you don't know what the "good spots" are in levels for defense. Chances are others do, so follow them. When they stop moving, hunker down and pick a corridor or doorway to watch.
-The point of KF is to not drop to zero HP. In L4D it's like "Haha, oh I went down can someone pick me up?" In KF it's "Uh, guys, I need....well shit, now I'm dead, am not respawning til the next wave, and I just lost 90% of my gear and all my money." In higher difficulties, 1 or 2 people dying the previous wave can sap enough of your firepower that you can't survive the following waves. It takes several rounds to build up cash for the best weapons and gear, and when you suddenly lose half that in wave 7, you feel it in wave 8 big time.
-Kiting is a last resort, an act of desperation to finish the wave. If you don't know what you're doing, you'll die in seconds, or in minutes eventually. If you DO know what you're doing, you'll bore the rest of your team to death. Literally, they'll get bored, leave the spot to find out where all the enemies are and BLARGH! Or they'll start killing stuff you're kiting, which causes new enemies to spawn, probably in front of you, and suddenly everyone is running around out in the open like fools. Don't kite, unless it's really absolutely necessary.
-As was mentioned, there's a few little rules that you learn by playing. Like not un-stunning Flesh Pounds or Skrakes. But there are others. Like healing yourself and team mates in literally the seconds between shooting guys. Learning which targets to prioritize, like always taking out Sirens because if they get in to scream range behind a wall of guys blocking your shots....GG. It's worth it to get bitten by a Clot or slashed by a Gorefast, if it meant a Siren didn't fire off on the whole group.
-The time between waves is when you have to be highly efficient. At first, follow your group, because the marker llliiieeessss and it's easy to get lost in some of the levels. If your group is slow and you don't make it back to the spot in time, at least you're not alone. Later on, once you learn the levels, you can start heading out to the trader early when there's only a handful of zeds left. That gives you extra time to get there, to think about what to buy, and to get back. If your group ends up not meeting you back at the spot...
-The hardest and most deadly time for players is when they are in transit between the Merchant and their defense spot and the wave has started. You will get it from every angle and if you get bogged down, people get spread out and cut off and chances are someone, maybe everyone, isn't going to survive the whole wave. Everyone is shooting for getting to the trader, getting what they need and getting back. Which makes it hard for a newbie to actually read stuff and figure things out at the Merchant. That's why you play easy difficulties to start with; spending all your time at the trader isn't such a big deal.
-As Metalax says below, be mindful that knowledgable players will make different decisions. The location of the trader sometimes necessitates finding a different spot. Sometimes the game forces your hand. If you missed that cue, or if you decided differently than your group, welcome to Meatville, Population: You. This doesn't happen super often but it does happen, and is an argument against running out to the trader by yourself. Level this is most likely to happen on? Bedlam.
-Etiquette with friendly fire on is very specific. In this KF and L4D are similar. Players are lethal to each other, in an already lethal game. With FF on, it's all about setting up avenues of fire. It is with no FF on too, but it's not as critical.
-That said, do NOT plant your butt out in front of the group and obscure everyone's lines of fire! This is my #1 pet peeve in KF and it will make me actively not support you so you die. (I will literally stand there and watch things eat you.) On Manor, for example, defending the tunnel, do NOT be that guy that stands at the mouth of the tunnel. Do NOT be the guy who steps out of the tunnel entirely and shoots stuff, only running back in when it's too hard to handle by yourself. Call it showboating, hogging all the glory for yourself, being an inconsiderate prick, whatever. In a game about defense, what you don't do is deprive everyone else of the chance to do their part. It's fucking BORING to sit there and watch you shoot stuff. Yes, it's easy now, but it won't be later and I don't want to spend 4 rounds falling asleep because you're impatient. Sit at the spot like everyone else and don't abuse the protection it offers. My friends and I are often the bulk of groups online, and when we're fed up, we simply hold our fire or even pick up and find a new spot. Point is, be considerate of the rest of your group, because benefiting from people's protection while actively denying them experience, kills and entertainment makes you a complete asshole.
-Do NOT strafe around, bunny hop or generally move a ton when defending. It's not necessary until the later waves and it's annoying as shit for your other team members. A lot of the favorite defense spots are also in fairly confined spaces. Movement is only necessary when things are going WRONG. When they're going right you should really not need to do it hardly ever. KF isn't a cover shooter where you constantly need to reposition to shoot new enemies.
-DO step backwards or strafe to avoid being attacked. Examples are: strafing to avoid a Husk's ranged attack, stepping backwards to avoid a Clot grab or a Gorefast slash, stepping backwards or sideways to avoid being puked on by a Bloat.
-DO move forwards to attack a Skrake with a shotgun. Again, this is just one of those tactical things you just end up learning is ok to do. When you unload both barrels of a double-barrel shotgun into a Skrake's face on hard or lower difficulty, it's usually a guaranteed kill. Most people know this, so it's expected for the shotgun guy to be the one that confronts Skrakes, and he gets a free pass to run out of the group and into its face to kill it.
-Likewise, when a Skrake is plunging a chainsaw into your guts or a Fleshpound is wailing on you, you have no choice BUT to run. Standing still isn't an option for anyone in those two specific cases. Everyone knows this, and no one is going to get bent out of shape if you start backpedaling, or turn and run and/or jump to stay alive.
KF is one of those games that seems incredibly hard at first, then actually pretty easy....which then goes back to hard once you truly grok everything that's going on. I'm not a max difficulty kind of gamer, but KF motivated me to play the game at its most intense and insane. And it can be nigh unbeatable unless your team is very well coordinated (and leveled.) I generally play hard difficulty, 10 waves. In our prime, we beat that 80% of the time. When we haven't played for months, we only win about 10% of the time. I generally don't play short or medium wave games, because while you get about the same amount of money...the # and types of enemies ramps up much faster and you often can't keep up with them in terms of upgrades. Long games make take more time, but the difficulty increase isn't as spikey...and you get to kill way, way more shit.
So KF is a game where success balances on a knife edge, difficulty and perk levels withstanding for the most part. KF can actually be really boring with a good team, because you're just mowing down waves of dudes in a corridor. But that's also one of the best spectacles of KF, is watching 30 guys just come apart under a hail of gunfire, in a way that makes L4D zombies seem almost laughable for how they fall and disappear after a few bullets. Shooting KF zombies is, dare I say it, a visceral experience. That said, the game is the most fun in the 2 waves where you're starting to fall apart, because it's when you have to be playing at your best. When things go wrong on Hard/Suicidal/Hell On Earth, they do so quickly, and 2 to 3 seconds is perhaps all the time you get to react to something.
A couple tips:
-Having your knife out makes you run faster. This is as close as you get to a sprint.
-KF's aiming has always struck me as a little different than other games. Your gun isn't coming out of your forehead, it's actually a little off to the side of you. Then there's also the lack of a reticule. This can make hip shooting a little tricky at first and make you want to use aim down sights for everything. Don't. Stay with it. Your brain just needs to adjust away from how most other shooters do it. Once you grok the aiming, you can pop the heads off most zeds while firing from the hip at ranges that wll surprise you. You have way better situational awareness when not staring down the barrel of your gun the whole wave, and it leaves you much more agile for avoiding attacks. This is how, for example, you can shoot a gorefast as it comes at you, and jump backwards right as it takes a swing.
-Try to use your pistol for the first 1 or 2 waves and focus on headshots. This is a good time to train yourself to "see the reticule." (A good time to level as Sharpshooter, as well.) The starting pistol is more than adequate for the first couple tiers of enemies, and buying ammo for your primary weapon at the start really eats up your available cash and can hobble you in later waves. You also make a little more money for a head shot (iirc), so it's a win-win-win-win. You'll know it's time to put the pistol away when guys are coming too fast for you to cleanly head shot them all.
-The first 3 levels of every class are brutal. Higher level people will kill stuff faster and you'll struggle a bit to meet your class objectives. Stick with it, or play solo easy to get some baseline kills started. Once your class weapon discounts and damage bonuses really start kicking in, things get much, much easier. Commando is the easiest and the fastest class to level, IMO, although needing to kill Stalkers to level is kind of a PITA and you start resenting other players for killing them, when you're doing the long hauls between 4, 5 and 6. Next easiest is Firebug, the only downside being its ammo, which if you want it to last the whole wave, means you don't a ton of damage, so you don't level that quickly. Medic isn't hard to level, but is pretty slow, especially when you're playing with experienced and level'd players. You start begging people to play recklessly. Support is about in the middle, because all of its requirements are doable...but the door-welding you have to be deliberate about, because it doesn't come up a ton unless someone is dedicated to babysitting a door. Demolitions also isn't hard, but takes a while due to their requirements relying on pricey weaponry they struggle to afford until they level up some, and then basically getting no decent class perk advantages that don't involve blowing shit up, on top of that. It's well worth the wait though. Sharpshooter can be tough depending on how good you are at sniping, and it takes many headshots at any rate. Berzerker is probably the hardest and the slowest to level, because you don't deal non-stop damage like you do with ranged weapons. Also, you die a lot.
-Berzerker is the hardest class to play right, it requires the most finesse and has the lowest chance of survival in a reasonably hard game. Functions best with a dedicated medic watching them. If there's any class that I think warrants reading a wiki article, it'd be Berzerker.
-Some levels are good for leveling up Support because you can weld a door near the spot and lock out half the enemies. Them continually wailing on it means you get crazy welding points if you just make your job keeping it shut. Unfortunately, it has the unfun effect of making the spawning inconsistent and it stretches out how long levels take. You can probably keep a door closed by yourself with no problems up until wave 6 or 7, at which point it will start taking two people to keep it up. Sometimes it's just easier to weld 1 door to 100% every wave, somewhere in the level, rather than trying to babysit one for an entire game. Or weld every door you safely can with the time available to you. (Has a lot of downsides, like screwing over a fleeing teammate, making trips to the Merchant harder and making the waves come very staggered for a while, as enemies are forced to bash down your randomly welded doors.) Anyways, after doing the above a few hundred times you'll get what you need. Leveling up Support's welding is just an unfun thing you have to find an expeditious way to get done, because welding just isn't tactically viable when you need it most, and generally just makes easy things take longer, so it never gets used.
So yeah. Good luck. I have a love/hate relationship with KF after so many hours, because it swings from "standing still and getting tunnel vision from aiming down sights for so long" to "Omg there's something behind me and I need to relo--I'm dead" pretty dramatically based on a lot of factors: level, difficulty, player count, wave count, perk levels, player experience and player style. Finding the sweet spot: not too easy but easy enough to win, can be tough sometimes in KF, even for a group of coordinated players.