"A DR full plate renders a lot of those themed characters inneffective. A d6 short sword can't pierce a DR 6 full plate.
There's where the problem arise.
Why is that a problem? Get a str bonus, weak fighter! get a masterwork weapon. Even better, pick up a longsword or warhammer to use against a heavily armored guy. Why on EARTH would short swords be considered good weapons to use against a knight?
Beyond that: when you have a system with armor as DR, some playstiles become better than other. A monk with two kamas that can attack 6 times for 1d6+6 might be on par with a raging barbarian who can charge for 2d6+40. But when they face a Plated Giant that happens to have DR 10, the monk is close to useless.
And this is bad...because?
The guy hitting lots of times for less damage should go against enemies that have less DR but more HP, or higher avoidance/defense. Imagine that, different PCs effective against different foes. That's a feature of DR, not a bug.
I want the monk to run away from me when I approach in glistening plate armor. That's why the barbarians should charge the heavy armored guy and kill him fast. It gives TWF and Two-handed weapon users different enemies to focus on optimally. I love it.
Give me DR please!!! even as a module. I HATE, HATE HATE the fact that hide armor wearers in 4e have the same AC as my paladin did. And that the rogue had a better AC. And so did the wizard. Suckage. Blowwy. Please, just make plate armor darn expensive, and half-plate and fullplate insanely expensive.
It'll give us a use for gold in mid levels when currently in D&D, if there isn't a magic shoppe in town or you play in a lower/rarer magic setting, masterwork mundane plate could give fighters and paladins something to look forward to purchasing for 10,000gp. If it's expensive, it'll be rarer. Most enemies wouldn't have it, and those that do, will usually be higher level, so you should be doing enough damage to hurt him and kill him eventually.
I think warhammers should bypass 1/2 the DR, but do less damage per die than swords and axes otherwise (and thus those are more effective against lower armored opponents). And why can't adamantine swords bypass DR too? Gives you more use for mundane metals used in masterwork swords.
Give us a reason to love you again, D&D!! something new and exciting to look forward to. DR will probably not work well unless it's baked into the system from day 1, sadly. At least put it in the PHB as an optional module. If it is, I will only play in games where that module is activated. I just like DR. It IS more realistic.
And if anyone tells me they'd rather a hammer blow to the head without a helmet on than with, I must go forth and puketh my disdain for such ...daft-itude