Alright Valve, here is what, in my opinion, you needs to give L4D to make it as replayable as TF2. Then, there's some optional thing that I think sound cool.
1) 4 more maps: The AI director is great. But, instead of making every match like a different level, it makes every time you play the same level have a degree of unpredictability. The sites still get old, and you are going to need more maps to keep the game feeling fresh and less an exercise in repetition. User generated content would also be great, and save you a lot of time.
2) New Zombies: The current 4 maps are fine tuned to the types of zombies, and that's fine. But Infected players are likely to feel a little simple after a while. So in the new maps, and perhaps give an option in the old ones, inject some variety. Replace some zombies with similar zombies that fill the same niche, but change the experience. Replace the smoker with another ranged zombie that slows the party down: perhaps it spits a gob of goo that damages, and slows players down. Or make a new Tank that exchanges some of its toughness for speed. Switch them around so all the niches you need to maintain balance are there, while diversifying the experience. Hold a contest, I guarantee you you will get thousands of ideas.
3) New weapons: The weapons feel sparse. Its understandable, with the gritty realism there going for, but that just means they are going to have to work extra hard to figure it out. The number one thing they need is Melee weapons. We have a zombie apocalypse game, and no hand to hand weapons: This is a problem. I recommend giving players the option to have a melee weapon as a sidearm, instead of a pistol: Arguably it would be more useful, but also more dangerous if your incaped. You even have a great reference gong for you. Make the first tier melee weapon a Crowbar. You gotta do it. Have the second tier be a chainsaw or fireaxe, depending on the realism your shooting for. If you have a melee weapon, it gives some great tactical choices. You can essentially have someone, "Tank" the zombies. Other possible options would be claymore mines, and shotgun/rifles with different amounts of penetration, damage, rate of fire, accuracy, etc, and remotely detonated explosives. I wanna say flamethrower, but really don't see it working out.
4) A Mall level. Nuff said.
Optional 1) Customization/ Character uniqueness: All the characters are the same, and this is a potential area with room for improvement. Unlocking a few cosmetic features would be easy, and breath some life into the game. Difficult to do, with so much effort put into making every character distinctive and identifiable, but some sunglasses or tattoos would give a sense of accomplishment. Also if every character had something that made them play a tiny bit differently, it would make them even more identifiable, and make players care about them even more. Maybe if Zoe could reload her pistols slightly faster, or Francis deals a bit more damage with his pistol whipping. A little detail could go a long way. Basically, something that makes me want to always play as Bill, or discover a nifty mechanic when they happen to play as Zoey for the first time.
2) Expanded crescendo events: it would be nice if players had more to do in an event then just press a button, and survive for a while. Make the environment more interactive. Maybe one person has to continuously turn a crank, and it starts falling if he has to turn around and deal with a zombie. Everyone else has to protect him. You could even have a different crescendo event where each player has a chance to shine individually. For example, one survivor has to move a crane to pick up the rest of the survivors and move them, or else they have to take a ridiculously more treacherous path. And then Francis happens to be better with heavy machinery, so he can operate it faster. Or maybe a security system needs to be hacked into to turn on guns and open doors, and Louise happens to have computer experience. Basically, different events where other survivors can do the task, but 1 survivor is better at it then the others, so everyone can feel like there special and in the spotlight. It doesn't even have to be to complex. Optional events would cool as well: Wait a few seconds to build a barricade, fight through a treacherous locker room to reach a room filled with pipe bombs and tier 2 weapons, etc.
3) Make the fixed guns better then completely useless: Seriously, when I am standing behind a mounted gun, and looking in the direction its facing, I still don't use it. I can still kill zombies just as easily, only I can turn more then a few degrees right and left, and I can move. The fixed guns are dumb.