Direct impacts to the head and asphyxiation (turn counting) are quite trivial design-wise. Blood loss is what makes combat more complex in the background. Each wound inflicted will add up to total blood loss during combat. If blood is nearly gone, you will fall unconscious and after that you die. Every body part will have a bleed factor, with arteries being obviously the most effective. Some small wounds will stop bleeding by themselves but there is no guarantee. Having small cuts all over your body will not be nearly as lethal as having lost an arm. (During combat that is, as infections can be quite deadly too). The constitution of a character is made up of his total amount of blood and his tissue's resistance. Yeah, body tissue is the next big deal.
Limbs and body parts
Following Dwarf Fortress' footsteps, I designed Humans in a way that includes their limbs, and for each limb there are several layers of tissue. Under the armor will be skin, muscle and bone. These are the basic ones. The really vulnerable parts such as organs or arteries are mostly layered between the muscle and the bone, but there is a limited chance to hit them. Some limbs will have special effects when they get hit: the ribcage for example will have a chance of stopping or not stopping thrust weapons with it's bones. Or the lower Belly, which is extremely exposed and vulnerable, will have no bones.
Weapon types and their effect
Weapons with different attack values (Cut, Bash, Point) will pass differently through each of these layers and cause different types of damage. Hammers for example are great for smashing Bones and internal organs, but they need to heavily mutilate their enemy until he really dies (let's ignore instadeath by skullcrushing for this example). Swords will have a quite linear damage distribution along all layers, which is predictable but not always what you need. Point weapons deal less damage in overall, but completely penetrate layers more easily. This will have a huge influence on the fighting style, as one needs to focus on lethal body parts to cause deadly wounds.