I like Left 4 Dead, but it's too fast-paced for me. The Dawn of the Dead-brand zombies don't actually feel like zombies...more like 28 Days "really pissed off regular person" types, cause when they get close they just hit you. The game would be scarier and much better if it was a little slower and the zombies actually acted like zombies.
Also, more weapon variety. I get tired of using the same weapons over and over again, and find it strange that the Uzi, Generic Pump-Shotgun, M16, Ruger and M12 shotgun are the only weapons currently in production in the Left 4 Dead universe. I'd like something more like what there is in Call of Duty, where you have a weapon or two and can modify it and such. That would be totally tits.