One of my hobbies is heavy weapons combat in the Society for Creative Anachronism. This is a martial art/sport that is meant to simulate medieval combat, with swords and maces and polearms. We use rattan (like solid bamboo) sticks to wallop each other, and wear armor to keep from getting concussions and broken limbs.
There is a combination of factors which contribute to "wounding" and "death." Some games address them; some don't. Here's how I get "killed" and how I try to avoid the same.
Not In the Face!
The best way to not take damage is to not get hit. You can either try to block a blow with your shield, your offhand weapon, or even your primary weapon. Most systems have some sort of rules for shield use and parrying. I'd say effective shield use is easier and better than most parrying, but anything that keeps your opponent's weapon from hitting you is Good.
More elementally, you can opt to not be there when the blow lands. This would be dodging, running away, what have you. Moving in armor allows for more mobility than some folks would think. Doing acrobatic stunts won't really help unless your GM gives out bonus points for cinematic style. While your hands are on the ground as you handspring away, your opponent will step in close and smash you in the ribs. Just stay on your feet and be fast. Modifiers to defense based on Speed, Dexterity, Agility, or whatever "speedy" stat your system uses are very appropriate.
Only a Flesh Wound
All right, your opponent slipped one past you. But did he hit you hard enough to penetrate your armor and hurt you?
Systems deal with this in various ways: Armor Class and Passive Defense are a few I can think of off the top of my head. It's rather important to note: it's really very, very hard to entirely miss someone in toe-to-toe combat. When you're doing the flavor text, it makes much more sense if "misses" hit shields or bounce off armor.
Aye, but here's the rub. Can a wimpy little kobold (or PC, for that matter) hit hard enough to ever cut through a suit of steel plates? Is there a certain Strength rating needed to penetrate certain kinds of armor? Are certain types of weapons ineffective versus certain armors? In the SCA, we clock each other but good with what are essentially clubs. When you're wearing a steel helmet, you don't fear much from a club swung at your head. I know someone who got a mild concussion, once, and someone who got a slice in his brow (his helmet bit him) but that's about it.
Of course, if you don't want to deal with it, you could always wave the magic flavor text wand and decree that wimpy but hitting attacks merely found their way through weak spots in the armor - joints, open helm faces, etc.
And then there's the reverse problem: an armor like chain mail that's not rigid provides very little defense against blunt weapons. It's great for stopping slashes and thrusts, but a goon with a club will smash up your ribs just the same. Some systems provide optional modifiers for certain armors against certain weapons types. If realistic battle is important to you, you might want to consider them.
I'm Dead, Jim
Combat is "deadly" and fast in the SCA. We make a simplifying assumption: if you hit me hard enough to breach my armor, you've also breached my skull. One good, solid hit to the head or torso kills. A single hit to an arm or leg disables it. We've got one Life Point, and if you get past our Active and Passive defenses, we "die".
For your game, it's a matter of style. Want realistic, gritty combat? Use systems where the PCs don't get a lot of Life Points (or whatever) and weapons do a lot of damage when they hit. If you prefer cinematic battles with long confrontations between villian and hero, with banter and good camera angles, use one where PCs can have Life Points many times more than the maximum damage caused by the weapons.
So that's my thoughts on taking damage in combat. Got a beef? Got a question? Go ahead and email me.