Then you get Pathfinder 1 which adds in the concept that certain pluses count as different materials as well.
To add context here, Pathfinder had an optional variant where each plus of magic item could also be considered a certain material for the purpose of bypassing DR. A +3 weapon counted as silver, +4 as adamantine, and +5 counted as evil/good/chaos/law (as needed). So a +5 weapon effectively bypassed pretty much every type of DR except epic.
Ultimately to me, DR serves three purposes:
- To create an interesting challenge to the players.
- As a "riff raff" negator, ensures that hordes of commoners can't just beat the monster.
- Weakens/nullifies summoned type creatures, who often don't have the same ability to negate DR as the party does.
The problem with damage resistance (half damage) is its often not that great for the second purpose. For example, as strong as the Tarrasque is, 1000 just absolute shlub archers (no bonuses or anything) will still do over 100 damage a round to it. 1000 archers might sound like a lot, but against a creature that a party of 20th level characters is supposed to have trouble against, its really not, any kingdom worth their salt should be able to supply that force easily against such a legendary monster.
The "damage threshold" concept is a much better fit for that purpose, it makes a lot of monsters "invincible" against your armies, which then requires specialized heroes to do the job. The damage threshold is also nice because unless its quite high, it doesn't come into play for a lot of PCs, and so you don't have to add in extra math. So I'm a big fan of damage thresholds on those key legendary type monsters.
The second issue is around an interesting challenge to the players, and this is where the binary problem comes in. Once players have a magic weapon (which in most games is going to happen), it throws off the math on a whole slew of creatures (as apparantely CRs do not assume magic weapons, and so high level creatures are supposedly much tougher than they are in actual practice). I much prefer gradients, monster X needs a +1 weapon, monster Y needs a +3. I am okay with material DRs if they are seperate from magic, I think magic or silver is silly. That said, I also think damage IMMUNITY needs to be rarer. The fact that werewolves are immune from nonsilver/magic damage is kind of crazy when you consider their CR, they should just be resistant. True immunity should be the purview of high CR monsters, where a party should be expected to have many ways to deal with it.
I also think the "slashing, piercing, and blugeoning" damage is quite a mouthful and very silly. Just add a new category of "weapon damage". Keeping your blugeoning or slashing for specific monsters like a skeleton that need them, but otherwise just say weapon damage. It will save a lot of real estate on the page considering how many monsters have it.
Lastly, I agree vulnerability needs a new look. Its so powerful that WOTC is terrified to use their own condition, and so it needs a tuning down. Later monster books have shown us a glimpse of what that might look like, where a vulnerability doesn't add more damage but applies a certain condition or penalty to the monster.