I am in the southern hemisphere, so it's always winter in the game. No matter how warmly I dress, I will always die of hypothermia. This is unrealistic.
When hitting a zombie with a melee weapon, it hardly ever hits unless I stand close enough to smell it's breath. This means when a zombie chases me, I will die, unless I climb up some stairs where it can;t reach me. This is unrealistic.
If I haven't had a drink for an hour in real life, I just get thirsty. In DayZ I'm dead. This is unrealistic.
If I haven't eaten for an hour in real life, I probably won't even notice. In DayZ I'd be dead. This is unrealistic.
In real life, I would have to run in the rain for about 15 minutes to be completely soaked through, but in DayZ this happens in 5 seconds of rain. Unrealistic.
If I fall off a roof in real life, I could sprain an ankle, maybe break a leg. In DayZ I'd be dead. Unrealistic.
Fixing all of these should be as easy as adjusting some of the values for how the character is affected, it shouldn't take reprogramming the game.