To depart from the Quincunx work that I'm currently pursuing, here's a completely different game mechanism.
I've found that a lot of people enjoy the concept of combat in games, but different people view the essence of physical conflict in different ways.
Some prefer an abstract series of die rolls, some like tactically placing their hits on their target's vulnerable areas; some want the die rolling to be fast, while others love an exceptional level of detail.
Without plugging the numbers into a computer program and letting it do rapid calculations, there is no real way to get good crunchy detail while also having the game progress at an action packed pace.
Either you get the adrenaline pumping with quick descriptions and abstract detail, or you really get into the minutiae of injuries and slow the combat down to "matrix-style" bullet-time.
Plenty of games try to find a balance between the extremes, some more successfully than others. I know of a few games that somehow manage to slow things right down to a crawl, but still give you little more than abstract detail as a result (Yes, White Wolf, I'm looking at you).
Like a lot of things simulated in RPGs, combat can be viewed in a manner that is either task based or resolution based.
Do you want to know specifically how something is done? (This tends to be the slow method)
Do you want to know the outcome once it's done? (This tends to be the fast method)
If you've ever been in a fight, you'll know that someone will try to hit you if you leave an opening, and you'll try to hit them if they leave an opening. Most boxing matches follow this pattern...as does fencing...and so do most other forms of fighting for that matter. You could set up for a strategic strike at a particular area, but if you're target realises that this is your aim, they'll just focus their defences on this area. Since they don't need to worry about defending other areas, that leaves them more able to expend their other energies in hitting you.
Sure there are people who'd argue against this analysis of combat, and they are free to generate their own systems of RPG physical conflict resolution. This combat mechanism reflects the notions I've described above.
Another thing that strikes me as odd about many combat systems in roleplaying games in the idea that you have a roll to hit someone followed by a second roll to see how much damage you do. One person could easily hit their opponent, only to do a minimum amount of damage; while another person could just manage to scrape against their opponent while doing huge damage. It doesn't make sense to me, and it slows things down. I like my combat rolls to cover both the hit and the damage in a single throw of the dice.
A single throw that shows how well the target has been hit and where the target left their opening. Beyond this point, the mechanisms for damage and prevention can follow a few paths that I'll offer.
Different games use different dice so I'll offer three options (d6, d10 and d20).
The d6 option is where I've picked up this mechanism, specifically from earlier versions of the miniatures battlegame Confrontation.
In this game, a hit roll consists of rolling 2 six sided dice. The high one determines damage, the low one determines hit location. You want both dice to roll well, because scoring a better hit locations factors into the final damage done to the victim. The distribution is basically as follows; 1 = Legs, 2 = Arms, 3 = Abdomen, 4 = Torso 5-6 = Head. Even though the head is hit on a 5 or 6, there is a remote chance of hitting it, because the low die has to roll this high (therefore meaning that both dice have to roll this high for a head hit to occur).
Final damage to a victim is calculated by taking the weapons strength, subtracting any armour value of the victim, then adding the high die result. This is compared to a chart to determine whether the victim has taken a light, medium or critical wound, or if they have been killed outright.
The die rolling is quick, and the system gives a nice outcome. Critical hit to the arm, light wound to the leg, medium wound to the abdomen...you get the idea. It gives a much better impression of the conflict than just saying "Lose 5 hit points". Sure it's not as detailed as the hundreds of tables in Rolemaster, but it doesn't take five minutes worth of die rolling and table lookups to perform a thre second swing.
Adapting the system to other dice sizes gives a better granularity of scale, without sacrificing the speed.
A d10 option could look like this; 1 = Lower Legs, 2 = Upper Legs, 3 = Weapon Arm, 4 = Defensive Arm, 5-6 = Abdomen, 7-8 = Torso, 9-10 = Head.
A d20 option could get really specific; 1 = Right Lower Leg, 2 = Left Lower Leg, 3 = Right Thigh, 4 = Left Thigh, 5 = Groin, 6 = Right Hand, 7 = Left Hand, 8 = Right Lower Arm, 9 = Left Lower Arm, 10 = Right Upper Arm/Shoulder, 11 = Left Upper Arm/Shoulder 12/13 = Belly, 14/15 = Torso, 16 = Neck, 17-18 = Head, 19-20 = Eyes specifically.
This level of detail allows for character to be protected by armour on specific parts of the body. Modifying the amount of damage that might be received from an attacker.
Different combat styles could also change the way numbers are assigned to the die. Certain combat styles might be more adept at hitting the central ass of the body, while others focus on disabling limbs.
Advanced combatants might be able to modify the low die up or down by a value of one or two, indicating that the combatant is more adept at placing their hits in vulnerable locations.
But the whole idea is that the strike takes only a single roll of dice to determine it's effects, and a single table reference to see what is actually done to the victim.
I've seen it work well in miniatures games, but so far haven't seen it work as well in a roleplaying game.