The greatest thing about Day Z is that you can't trust other players because you don't know what their intentions are -- it's the tension of not knowing if someone will be friendly or not, and trying to decide how you should handle the situation. With people shooting on sight, you don't have time to feel any tension because the moment of conflict goes by in a flash without any sort of build-up. With everyone taking a "shoot on sight" stance for self-defense, assuming that everyone is a bandit and would do the same to you, you lose out on the psychological dilemma and the game essentially becomes a simple, straightforward PVP shooter, when it has the potential to be so much more nuanced than that if people didn't simply shoot on sight.
