Armor has always been a weird one, and it is for most fantasy systems.
To me the best and really only solution that maintains speed of play is to use something like temp hit points, i.e., the big suit of metal armor absorbs X damage first.
Individual-strike damage reductuion (DR) brings with it a host of corollary problems, the biggest of which is it completely shuts down foes who, while skilled, don't do much damage with any one blow and instead do their killing as death by a thousand cuts.
Aggregate DR, e.g. this armour will absorb the first 36 total points of damage its wearer would take after which damage goes through as normal, implies the armour is falling apart after that threshold is reached and needs to be replaced. Somehow I don't think that's the intent here.
Because other systems that go further by incorporating tactical rules to distribute damage and other things like that just go too far in terms of slowing the game down, in my opinion.
Fair enough. D&D has also never had a called-shots system ("I intentionally strike at his weapon hand so he can't wield as effectively") which most of the time is fine, but corner cases where attacking and-or damaging just one part of the body makes far more in-fiction sense are frequent enough that some rules to cover them would be helpful. For example:
--- you stick your hand into the niche in the wall and the niche is a trap - take 12 points damage as it closes on your hand
--- you notice your foe is limping and want to specifically strike at the weaker leg to slow him down futrher
--- if all I can see is her foot, that's what I'm gonna shoot. Ha! 20! Take 14 points to the foot, you!
Personally, I just end up winging these through narration as DM; for example someone who just got shot in the foot for 14 points damage isn't going to be moving at full speed for a while. But some guidelines would still be handy.
D&D out of the box gets it close to right. Pretty good compromise overall. Easy math, fast gameplay (critically important but often underrated), not completely absurd in terms of probabilities and physics.
Yeah, it's good enough to be good enough - but it could always be better.
