3 different things to say about the actual health system.

First :
Its not always realistic and work too much on arcade way, if we put several bullets on the legs on someone (npc or player) he will die, heavy bleeding can kill, but repeated bullet on hit on non vital area can't kill simply like that, its just a "hearth bar/point" system who are wrong and unrealistic.
Player and NPC must be affected by the pain and injury like broken legs who prevent to move or make them move slow and limp, also some vital area are not correctly represented, some bullet hit in the head don't kill, in real life it kill or at least neutralize completely the soldier who will not be able to fight like a regular soldier, but he can still affect the fight even with a sacrifice action by waiting the enemy to be next to him before unpin a grenade, go on turret, spot target or anything else, depending of a lot of things like his wound and his mobility etc.
Heart and spine hit are not mortal in actual gameplay.
Vital, painful and mobility affected area must be definite separately.
Games like Sniper Elite V2 even simulate skeleton brake and organ damage, no one ask for a 100% body simulation but a realistic basic one.
Things like bleeding must also be take seriously in the equation, less blood = weaker, less concentration, more affected by recoil, can carry less, fatigue faster and can pass out and if the bleeding are not fixed die.
If we are heavily wounded we also must be more subject to be more sensible to truck/car heavy drive, a guy in a bad condition can die if the driver take too much jump or heavy vibration.
And about it the damage that the player took inside a vehicle are unrealistic, when we hit a wall at 20Km/h we are hurt but we are not when quadbike flip, we are not even ejected.

Second :
Medic must be improved, more realistic healing, it took over a minute for fix some heavy wound on the battlefield, it can even fail sometimes, possibility for player to do it automatically like bot or actually (simple select threat wound option), or for those who don't want to wait without doing something during the wait time, have a "medical" interface of simple things and manually do it, not too much complicate but enough complex, morphine who kill pain but who is not the best way to lets someone fight must be implemented with effect, overdose can kill, pain don't help but get high don't help neither, a balance must be find by the medic player who will adjust the quantity depending of the "medical interface".
Things like fix a broken legs, give painkiller, stop bleeding, extract a bullet, or stabilize a player for medical extraction (who can become a secondary objective of the mission) and other things, it can also bring things like poison and chemical warfare.
Anyway Arma is a game where player like to be specialized in a task.

Third :
Player interface, it is totally realistic to have an interface who show the body and the wounded area, in real life we can clearly and easily identified where we feel a pain if we have a broken legs or anything else, in the game with the effect caused by bullet suppression in fact i don't even know when i'm hit or not, its not realistic.
We must have an indicator, in real life if we bleed we feel and see it, in a game we can't feel it and graphic even in the beautiful Arma 3 are not yet enough for really see and identified this, like the new stance indicator who will coming for the beta, an injury indicator hidden until we are hurt must be present, we have a 100% precise interface and if the enhanced medic interface that player interact with is implement the player can help the medic by give him on tchat/VOIP or TS indication for him to have more precision in his work, more precision treatment he have, more success chance to be threat correctly we have, this can also implement a fatigue bar cause in real life we really feel how exhaust we are and no visual effect or sound can replace it and an abstract cursor (with a melted color for make us have a basic fatigue info without be too much precise).

All this 3 things can really change the game and make it more realistic, more intuitive and more interesting in the same time.