You'd have to change the entire game, though. Slow zombies are scary because they build suspense. They build suspense because you can feel your life slowly being whittled with each bullet you fire hopelessly into the encroaching mob of rotting flesh. The point isn't to have a mass of slow fat 'Kwan zombies for you to fire a chaingun at until they all execute their Doom 1-era death animations. You're supposed to be the underpowered, unskilled, frightened, and hopelessly outmatched survivor trying to do whatever he can to last as long as he can.
Essentially that's why Zombie Panic was so good to anyone who bothered to play it for more than 5 minutes; it was actually fun to lose. You could tell you were going to die several minutes in advance, so that when you did have miraculous escapes- often running down a hallway frantically screaming on your mic to hold the door open for a few more seconds, firing your last 10 pistol rounds into the carrier form on your tail- it felt so much more rewarding. You always *just barely* surviving, so that when you died it didn't feel cheap. Why? Because you knew what was happening, you were fully aware of the choices you were making. You weren't just shooting zombies and hoping that you survived, you were planning routes, watching ammo counters, remembering ammo spawns, looking for flashlight beams, all while trying to either stay silent and unnoticed or unload as much firepower into the approaching mass of zombies as possible. The slower pace allowed for more interesting player interaction. It did this by avoiding the consolized view of player interaction; which is essentially pushing buttons at specific times to accomplish a task, what varies is the frequency of the button pushing and the location.
Zombie Panic:
You spawn. You're in a fucking closet, with a pistol, twenty rounds, a crowbar, and it's pitch black.
You leave the closet, do you go left or right? What do you remember that spawns down there? Do you want to try and go deeper towards the zombie spawn to grab a rifle or a magnum? Do you want to try and group up with players? Where? When?
So you got a weapon. Got any ammo? Where is the ammo? Hear any zombies coming yet? If they do, how are you gonna stop them? If you panic, where will you run to? Do you see any other players yet? Where are they going? How fast can you make it there? Do they have ammo you can use?
You met up with some guys, you're headed across the map to barricade a house. All of a sudden three zombies and a carrier hop out and block your path, stumbling towards you. Where do you go now? Will you stay together or split up? Do you have weapons or not? Fight? Run? How good are the other players, will they chicken out and run? What's the next closest safe spot? Will it remain safe forever?
You get to a small shitty safehouse with one other player, barricade lightly, and shut off your flashlight. Do you stay here or try to make it into the strong fortress with 5 players in it, all stocked with ammo. How far away is it? How much health do I have? Stamina? Can I carry this shotgun and still make it? That rifle? If I do decide to stay here: Did any zombies see me enter? Where would they try to attack from? What's my escape plan if they do? If I panic, what weapon should I take with me?
Another player tells you to get to the safehouse before they barricade. Do you go? How does it look outside? Flashlight or no flashlight? What should I tell him over the mic? Which entrance is easiest to go through? And so on and so on until you die or you survive.
Left 4 Dead:
You spawn. Do you pick a shotgun or a rifle? You run down the street, shoot some zombies. Horde comes, one guy falls down mid-fight. Do you run back and let him bleed out, potentially saving your own life, or try to fight and save him before he dies? Even then, he'll be waiting in a closet several hundred feet away. So he dies. Take his gun, or no? You find pills, do you take them (24 permanent HP) or does your teammate take them (40 temp HP)? Someone falls, you help them up. Someone get's grabbed by a smoker, you fire a few shots in their direction. The speed of the zombies coupled with the lack of barricading disallows any true tactical thinking. When you die it's almost impossible to go back and analyze your mistakes because you never really made any decisions. You got some baddies thrown at you and you shot them before they reduced your HP to zero. If you die, 90% of the time it is because they reduced your HP to zero before you shot at them enough times to send them up to the land of multi-headed dicks. The other 10% is when your team mates are clinically retarded. It's like comparing the game tree complexity of Chess to Checkers. It's just on an entirely different level altogether.
Is it fun? Of course it is; put 4 people together, give them guns and beer, and they will have a blast. That formula has been working since beer and guns were invented (beer and arrows, too). It's not fun because it's interesting, though. It's fun because you're together with four friends and alcoholic beverages.
