What the hell is it with you people and Melee weapons? You have one. It's called the butt of your gun. I don't see what your obsession is with wanting to get close to zombies. They want to eat you. You don't want to be near them to kill them.
My own personal improvements to L4D would include a few more different styles of campaigns; modes where you would have to defend a structure, with a CS:S style buying system that could be used to buy guns, ammo, grenades, health kits, etc, as well as repairs and reinforcements to the house. Eventually a tank would be spawned and you'd have to go and move into another structure. This option would always be available in case of being overrun, but a tank would force it on the players, as well, it's a fucking tank. Money could also be spent to build walkways onto other buildings.
I would alos mkae the melee/shove attack a bit more powerful. At this point it seems to take four or five smacks to kill something, and while it does a good job of keeping stuff off of you, there is no decent means of silently dispatching a zombie. I would either make shoves more powerful or add another melee attack that would only affect one enemy at a time, but would be able to kill them much quicker.
I'd also want another pistol, that would be bigger and more powerful, compensated by you only being able to carry one of them, as well as it's smaller magazine size.
Random, NPC survivors. You'd find them besieged by zombies or inside a barricaded structure (which would be a normal item closet that became barricaded. They might need you to give them a health kit or something, but then you'd have another person helping you out. It'd also be cool if you could find maps written on the walls of safe houses, they could be generated along with the other stuff the AI director makes up, and would point you in the direction of where you needed to go, as well as the occasional weapons 'n' ammo cache.
EDIT:
After reading someone mention Create-A-Character, while we'd all like to be our ownselves trying to survive in the zombie apocalypse, I think Valve went and put a lot of work into the characters for a reason.
Also, to the people demanding more weapons: The weapons are fine. They're all there to serve a purpose, and we do not fourteen different kinds of shotguns, submachine guns, pistols and assault rifles. This is not Call of Duty or Rainbow Six where you have seven different guns that serve the same purpose. Each weapon was designed to serve a function, and was then modeled after a real gun, with slightly higher than normal ammunition capacities.
