It's always had infinite ammo. And yes, I'm refering to the automatic pistol. They're still perfectly capable of killing zombies though, especially if you're dual-wielding.
Yes, you can do more damage with bigger guns, but you never panic about running out of ammo in Left 4 Dead.
Personally, I see L4D as being in the Serious Sam genre. Lots of guns, lots of ammo, lots of health, and killing lots and lots of bad guys.
Sure, zombies are supposed to have high numbers, and sure, it's likely to be hard on max difficulty, but sneaking around to avoid the horde because you've got no ammo is an exhiting gaming experience that L4D isn't going to manage IMO.
And okay, it might be different for 2-player console kiddy-games
But in the real man's world of FPS gaming, you often have to interact with strangers. Some of them may even say mean words, you'll just have to deal with it.
Some of us even like meeting new people. Hell, I'm disappointed that it's only 4 people in your party. I wonder how many people can play as zombies...