It's there to make your own theories, like with the special infected, I think it's to do with the lifestyles of the people before they turned. Smokers could of been stoners or something, and the chemicals in the blood altered how the infection changed them. Tanks were body builders, meaning the infection had excess muscle tissue to mutate. Boomers were fat bastards, giving the infection a lot of meat to work with. Hunters look homeless, so maybe it was the lifestyle with the dirt and grime that caused a mutation.
Really it's better if it's not explained, but I do agree that showing how they meet might have been an idea, to actually connect the characters, though the opening implies that Bill and Francis have known each other a while.
Also, I agree that different characters should have different perks and flaws, for example:
Louis could reload faster thanks to being good with his hands, but suffer a marginal accuracy drop since he has no weapons training.
Bill, being a war veteran could be a little more accurate with weapons, yet in his old age have a slightly slower beatback rate.
Francis could be slightly more profficient with his beatback, while taking longer to heal as a result of having little first aid experience.
Zoey could perhaps have medical training enabling her to heal faster or for more health, but take longer to get up after being incapacitated due to not being very durable.
