Reading through the lazy player thread, another difference between milsim and skirmishing occurs to me.
In many skirmishes, players will rush objectives or try bold moves, knowing that failure involves a short walk to regen and they are back in the game a few minutes later.
In milsim, regen is deliberately made to be an effort - usually a long walk or at least medic rules with bleedout time. This is not done to be mean or inspire some kind of hardcore experience.
There is nothing like being scared of being shot.
Its a natural emotion in a real action, for obvious reasons.
Long regens help to add a degree of realism.
That heart thumping fear and inevitable adrenaline rush when a firefight kicks off and you are, both trying to conserve limited ammunition by short bursts of auto or better still, semi and you know that taking a round will put you out of the game long enough to at least miss that firefight.
You often read about hi-cap fests with players trying to get as many balls in the air as possible to maximise their chance of winning. Milsim players using mid caps or sometimes even real steel limit low caps will conserve ammo and pick their shots, knowing that any prolonged action will have to be punctuated by time sapping mag changes.