Someone's put together a Left 4 Dead mod that aims for more realistic gameplay but definitely ratchets up the difficulty. It does pack a nifty laser sight however.
A modder by the handle Doku found the original a little too arcadey and set to putting the survival back into survival horror. Among the fun he delivers to the PC version:
• New weapon sounds (Most made by Doku but with others grabbed from various soundpacks).
• New weapon scripts - Adjusted accuracy, recoil, rate of fire, clip sizes etc.
• Weapon melee less effective.
• No survivor bots.
• Slower survivors and zombies - to change the pace of the game and make it feel more tense.
• Higher density of zombies on the maps, more frequent hordes/ambushes.
• No special zombies (apart from tweaked tanks for the finales).
• Headshots only to kill zombies (apart from molotovs / pipebombs etc).
• Increased damage when hit from behind - you must always be aware of you surroundings, don't turn you back on that unchecked room
• No in-game music - You have to indentify incoming hordes by the sounds of the zombies.
• All maps tweaked (apart from part 4 of the blood harvest campaign)
• Removed weapon,ammo & health pack spawns from safe rooms. Weapons and health packs are now limited and scattered around the maps, so you have to search for them. There are no ammo points in the map so if your primary gun runs out you'll need to find another. However everyone still has infinate pistol (although with smaller clip and less accuracy).
• HUD completely revised - It now only shows your own health, no crosshair/inventory or teammate statistics (If you equip first aid then you can see your teammates health when you aim at them).
• Added in-game laser dot - Useful for close to medium range. This emphasises the sniper rifle as a profficient long range weapon as its the only gun with an effective long range aiming mechanism.
• Removed flashlight - This adds much more tension to darker areas of the map, especially when you need to search for weapons or first aid... Be careful in the dark...
• Various other smaller tweaks.
I don't have the PC version (don't have a PC) but if I did I'd be willing to give this at least a few tries. It sounds more like it was written for a singleplayer experience, although with no survivor bots, you're not gonna last long.
Doku's Survival Mod for Left 4 Dead [L4DMODS.com]