To address
@Celebrim 's post from above, I wasn't planning to just bolt DR onto an existing system exactly, but rather use a somewhat fuzzy OSR d20 base and then redesign or add whatever parts need it to make the system work. Ideally, it's a core mechanic with all the specific design decisions that entails, not just a widget.
Someone mentioned it, but I feel like the Conan system was one of the few times I've read the rules of a game with armor as DR and felt that it was actually play tested. Basically, I'm warning you that as intuitive as this idea is, the fact that it has never really caught on despite many attempts by systems to implement it should be a warning that it's tougher to get good results in play out of it than you might think.
I think of a system like GURPS, that I do have a decent amount of experience in, and the guiding principle in GURPS design was always make a system that is intuitive process to reality. GURPS has armor as DR. And the problem with that is even if your guiding principles were intuitive to reality (armor intuitively reduces the damage from a blow) it doesn't necessarily mean that the resulting combat will look anything like real combat. A great example of this is "Dwarf Fortress" that pursued this intuitive approach to simulation far beyond what you can do in a PnP game, and still ended up with a result that was massively unrealistic and even comical in its ludicrous cartoonish outcomes. The transcript of a Dwarf Fortress fight is rarely either heroic or believable.
The other thing I tend to find is that this approach doesn't necessarily lead to more fun. That is the combat more fun to engage in if the process is more intuitive to reality? Picking on GURPS things seem to work OK at first, but the longer you play it the less engrossing GURPS combat gets. A fight between two expert duelists especially in older versions of GURPS comes down to waiting for one side or the other to get randomly lucky, after which it immediately death spirals. Most attempts to fix this involved adopting more and more intuitive to reality processes to add complexity, but eventually that complexity became to overwhelming to play test. My expectation though is that if you did play it, you'd end up with the "Drawf Fortress" problem where playing out a fight in the game obviously wouldn't be fun or dramatic.
My suspicion is that things can be made to work OK if you are dealing with gritty combat that engages say someone really into HEMA, but where your combat system is so realistic that it basically deters you from having a game around combat since on average no PC will survive more than about six combats before hit with a random lethal blow. Or else, on the other spectrum, the PC's game the system and the DM keeps his padded gloves on and the PC's run around like nigh invulnerable tanks safely protected behind armor as plot armor where nothing has a real chance of both hitting and penetrating the PC's armor before being turned into chopped meat.
To be honest, I'm starting to have that problem in my Star Wars D6 game with its "realistic" wound tracks rather than hit points, in as much as we have an action focused game but the math is serving mostly to hide from the players how rarely combat has any chance to alter the results. Star Wars doesn't have armor as DR per se, because it doesn't have hit points, but it does mitigate damage through a soak roll that works similar to having high DR but low hit points. We're doing the ritual of combat, but statistically often mooks have like a 1 in millions chance of doing anything meaningful to a PC. But if the chance that the mook does something meaningful isn't very close to 0, then the math makes the chance of instant death non-trivial. And you have death spirals. So combat is either in some sense pointless or extremely lethal and there isn't much in between.
I suppose this is realistic at some level in as much as in a war zone, it only takes one bullet with your name on it. But does it make for a good game? And as a GM, it makes it really hard to prep material that makes for good fights.
