Also, critical hits kind of take this into effect. It's very easy to penetrate armor with a rapier, so it has a high crit, but a crit from an axe has a better chance of taking off a limb no matter what you are wearing. D&D, no matter it's edition, has always had a way of making things work without always making sense. It's a kind of magic...
But a quick and dirty way to do it would be to give light, medium, and heavy armors a small, flat DR. Something like 1,2,3, or 2,3,4, respectivly. Piercing weapons would negate this on light and medium. Slashing would negate light. Blugeoning would negate Heavy. Not the most accurate way, I know, but it's quick and easy.
