It might be the fact that you get "locked in" to reloading, which seems like an intentional thematic choice. If you start to reload, only to get grabbed from behind, you can't just shove-lock the zombie until you finish reloading. You're actually vulnerable until you get that magazine in. Taking damage also disrupts your vision for about a second, and prevents you from using ironsights. I could go on about the tactical implications of this, but essentially it serves the purpose of L4D's pin system (except that fleeing is an option, *if* you have a secured escape route - which usually means someone keeping an eye on the corridors/field behind you).
The game is honestly very different from L4D. I like both, but KF is less about split-second shooting and more about tactically managing the field.
A typical wave 6/10 on West London often goes like this:
Your team of four exits the shop and walks into the street. A sniper, a commando, a berserker, and you have a shotgun (Support). The street is long and open, making it a great place to defend with careful, accurate shots. Except, there's soon skittering and creepy laughter from the nearby rooftops. The street is a trap: early-invisible stalkers and hopping crawlers will continually attack from the sky, making it impossible to focus on the tougher zed hoard advancing from the only two escape routes.
So instead you run for the bridge tunnel to the left (the tunnel to the right works too, but has an annoying double decker bus providing the hoard cover). A few crawlers jump down. Very short, hopping around on 8 legs, they're weak but hard to hit. Your shotgun works wonders, you even get a couple as it falls from above. The commando helps with his automatic assault rifles and ability to see enemy health bars... which allows him to spot the invisible stalkers with ease. Still, you bump into something invisible just before reaching the tunnel. You reflexively take a few steps back as the stalker uncloaks and swipes at the ground where you stood, cackling madly. They're fragile like crawlers, so you dispatch it with a couple pistol shots without taking time to aim.
You finally reach the tunnel. The berserker got here first, both due to his speed boost and his hatred for crawlers. Berserkers are a great all-around class: Fast, tough, unable to be grabbed by clots, and able to swing their swords at lightning speed. Their only weaknesses are crawlers, (due to their shortness and constant hop-attacks) and running out of space to kite bigger enemies. The berserker already looks uneasy in this rather cramped tunnel, but at least it protects from the rain of crawlers. A pile of clots and gorefasts lies on the ground, all without their heads.
You all set up on top of the cars. The sniper choose which direction to cover (away from the trap-street) knowing that someone will support him. Any of you three (berserker, commando, support-shotgun) could do it, but the commando can see stalkers so he goes. Your shotguns and the berserker's swords mean that stalkers only ever get one hit on you, if that.
Things go well. The commando focuses on stalkers, crawlers, and whatever else he has time for. The sniper shoots the slower stuff, like bloats and husks, until a scrake arrives. Scrakes are walking tanks with chainsaws. They're specifically strong against explosions - I've seen one receive 4-5 M32 grenades to the face, only for it to emerge from the dust cloud enraged. That's the other thing: they're very slow and low-priority until about half health, at which point they flip out and charge into melee. The commando knows this, and knows that scrakes are basically a giant "F U" to his bullet-spray based class, but he can also see the thing's health bar. He wittles its health down about a quarter, then the sniper finishes it with a crossbow bolt between the eyes.
On the other side you and the berserker are also doing well. He keeps going out of the tunnel a little for the extra space, but retreats back in periodically to heal up. His armor is wrecked, but (unlike medics) most of his damage resistance is innate. You shoot the crawlers which keep going for him, allowing him to take on almost everything else. Whenever he falls back, you fire into the center of the hoard to thin it out (bizarrely, shotgun pellets all have penetration. No, it doesn't make any sense, but it means shotguns are great at wrecking hoards). You keep an eye on your ammo, and see you're getting low on shells.
Just then another scrake approaches! The berserker charges at it, ignoring the other enemies. (Scrakes are natural prey for berserkers, and I have to admit I've often gotten tunnel vision when pursuing one.) He swings at its face, every hit stunning the scrake for half a second - exactly as often as he swings. The rest of the hoard is tearing at him, though. You use some of your last shells to shoot the gorefasts and clots around him, but a couple of pellets hit the scrake. Somehow (I don't understand why this happens, except that the bond between scrake and berserker is holy and most not be disturbed) this snaps the scrake out of stun, and it swings its chainsaw into the berserker right before it succumbs to its wounds. The berserker is at about 10% health, which means he can barely see or hear until he's healed. In desperation he starts healing only a few meters from the hoard, which slows him to a walk - and a crawler leaps from behind a car, finishing him. Oops.
Fortunately the wave is nearly over. You shoot the couple crawlers nomming on your friends corpse, then exit the tunnel looking for stragglers. Instead you find yourself face-to-face with the grinning visage of a 10 foot tall, metal-clad giant. A glowing yellow energy core is in his chest. He bats at you with the studded meat tenderizers he has for hands, knocking you back. Fortunately he's still calm, striding into the tunnel with a metallic chuckle. You slam "FP" into the chat before turning around and running. You're pretty wounded, and this is a job for the sniper...
There's the twang of the crossbow, and a roar of anger and whirring cybernetics. Your overconfident sniper "friend" didn't wait for you to get clear, and the dumbass shot the thing in the shoulder. You leap over a car, turning - of *course* it's coming after *you*. Glowing bright red, meat tenderizers spinning, and faster than a charging bear. Your AA12 is half empty, and there's no time to reload, but at this range... it might work.
Er so yeah, not really like L4D. Both really great games, I have a few hundred hours in each.