When it comes to armor as DR, I disagree. It make the game more complex and screws around with balance. Let's take two different monsters, a hydra and a frost giant.
A hydra starts with 5 heads and can grow more. It's average damage is 10 points but it compensates by getting a lot of attacks. Compare that to a frost giant which does 25 point of damage on a hit but only gets 2 hits. Both are CR 8.
In the current system, ignoring crits, both do a maximum 50 points of damage turn. With DR? If you have a DR 10, the hydra will do 0 damage, the frost giant is getting 30. DR 5? Hydra does 25, the giant 40. I don't see how to fix that without completely changing how we handle attacks and damage.
Depends on how you run hit points. A friend of mine designed his own system back in the late 70's and still tweaks it, but this is how he runs hit points, armor and armor class.
Armor class is a function of your size, agility, dexterity and experience. So a big guy is going to be easier to hit than a halfling, and your dex and agility(he split dex) will add to that, as will a small increase in percentage when you level. So if you start with 32%, your AC is 7(still uses 1e low is better). If you gain a few levels and hit 40%, AC drops to 6.
Now let's assume you are hit. First you have stamina, which function like non-meat hit points. You can lose stamina from staying up 3 days since tired = low stamina, or from being hit, or from a bumpy fall down a hill. He also has PBP(Personal Body Points) which are based on constitution and size. Big guys are easier to hit, but have more physicality so more personal body points.
If a hit reduces your stamina to 0 or lower, it has now impacted your body and is doing physical damage. He rolls on a chart and lets say it hits you in the arm. He did 10 damage and you had 7 stamina left, so 3 went into the arm. If you had 8 PBP in the arm, you are now at 5 and your arm is bleeding and will take time to heal. Stamina comes back fairly quickly with rest. Critical hits not only do increased damage, but they bypass stamina, so you don't want to get hit by one.
Another function of PBP is that they are painful and hamper you, reducing stamina. So a first level fighter with 13 hit points that took those 3 PBP in the arm would be down 6 stamina until healing starts and they get some points back. Each PBP point of damage reduces stamina by 2 until healed. If your body area, let's say arm is reduced to 0, it's useless. If it is reduced to minus it's full value, it has been cut off, crushed to the point where amputation is necessary, dead if it's your head, etc.
Armor is DR in his game. Starting with clothing with provides 1. It doesn't do much, but it does more than bare skin. Full plate is 16, but very, very, very rare and expensive. Armor also doesn't always cover all of your arms and legs, as a lot of leather and chain armors stop about halfway down both, so his charts differentiate between upper and lower extremities. Get hit in the lower armor or leg and your armor isn't going to help you.