I love these games and look forward to more from you. No need to echo what the majority have raved about, but I will suggest for possible future installments that your character be customizable. When you're hopping around and blasting away with fellow survivors, it can be easy to lose track of your character since the looks are similar.
Also... Where the hell are the women? Okay, for the sake of argument, where are the obviously defined women? One could argue that some random survivors are female, but every character named is a guy and references to others are always "he" and "him"; A minor but legit gripe for an otherwise very fun game.