Im not sure why you guys are so hung up on mics, I have gotten 10 bronze, 4 silver and 3 gold so far, very very few of those games had mic users playing, and I dont use a mic myself, I love when someone with skill has a mic, but thats a 50/50 right there, many mic users just yell when incapped or bark out insane newbish orders, while the visual and audio clues are sufficient to alert you to anything going on.
I do however prefer someone with a mic over someone without one, mostly because I like the banter, but not in the magnitude of kicking anyone for not having a mic, it just isnt all that important in L4D if people know what they are doing to begin with.