EDIT: ninja'd by an admin.Fixed it. You know, I've never seen damage reduction in 5e, but I saw it in 3e. A pretty simple fix to the 3e version is to require minimum damage - so your example hydra would still do damage if the DM
fudgedrolled high enough.
But I still think it has a ripple effect. If my monk fights someone with DR, suddenly hitting a lot is significantly worse than the rogue who has a single (sneak) attack. In addition IIRC, DR in 3E was pretty low. Wasn't it something like 2 points? I always thought it was more headache than it was worth. In addition it's far different from the proposed 8 points of DR suggested in the post I was quoting.
I will be the first to admit that armor is pretty FUBARed. Someone in high quality plate should be very hard to damage unless you wrestle them to the ground and stab them in the face. But that wouldn't work for D&D combat. So with a high DR you'd have to have some way to bypass it, but then in D&D people can take multiple hits ... and it would just be a mess. IMHO of course.