In Left 4 Dead, many people have brought forth explanations as to why the special infected mutated the way they do. Here's my theories, and I think you'll find them derivative.
The Hunter - The survivors call him a 'jogger' most of the time, and his behavior is clearly inspired by parkour. Keeping in theme of irony with the Boomer being a health nut, I think he was a tech-oriented guy. He had the latest PDA's, cell phones, bluetooths, and GPS's. Things that kept him connected to the world through wires and airwaves. And you see more techies wearing black sweatshirts than sweatpants, trying to look badass. Why do I think this? Look at the Hunter's eyes. He has none. He's regressed to a natural predator, using his ears and nose to detect prey (plus some think his constant screaming is a form of echo-location).
The Smoker - This one is the hardest to think of. My first instinct is that he is some kind of lech. He's got a light jacket, jeans, sneakers. Kinda tall (although that may be due to mutation). I get the sense that he's a guy who spends all day in his house eating salt & vinegar potato chips, playing WoW, and searching for porn. Maybe he's victim to his physical vices, like sloth, lust, gluttony. It would be ironic that he became someone who could latch onto others and pull them into his clutches.
The Tank - A body-builder? No, but I think the Tank did start with some kind of anger issues. Evidence for this is the frustration meter. The Tank is meant to attack fast and furious. If you don't, the computer takes over and does it for you. So whether he was muscular or not doesn't matter. It's the rage that consumed him. You can see it's consuming him by his head being absorbed into his body.
And as for The Witch? There are so many theories floating around about her, I think everything's been said about her that needs to be said. I don't need to comment on her. But I think she's the most interesting "character" in the Left 4 Dead canon. Go look at the TV Tropes page for Left 4 Dead to see what I mean.