After playing this all weekend, i've come to the conclusion that what L4D really needs is the following:
1) More weapon variety (I could have sworn at the end of the demo it said there were more weapons in the full release... i've not seen any...).
There reallyneeds to be some good mele weapons in there as well.
2) Less ammo dumps/no ammo dumps. Yup you read that right, there is too much ammo in this game, even on Advanced I very very rarely run out of ammo. I want to be fighting for my survival only for my weapon to click dry and have to fall back to running around screaming blasting zombies with pistols. It'd add a bit more tension to the game.
3) Weapon pickups scattered through the levels as well as in safehouses, I want to pick a shotty up, it only have say 24 rounds, I can either use that and then fall back to pistols, or use pistols till I need the extra grunt, and when its run out, discard it until I come across a new weapon.
Why? Because as it stands I can pick one gun up at the start of the game and never change it for the entire campaign. It'd mix things up a bit if you were forced to use different guns as you found them, make you switch your tactics as you found new weapons.
It'd add more suspense as you are always on the verge of running out, and constantly on the lookout for a new gun.