The main implication of DR is reducing the power of spellcasting without inlcuding the RNG penalty to utility glyphs. It would also annoy wizards, since spell cost reduction doesn't help if you can't deal any damage with your Pisorff/Burndayraz.
It would also differentiate max mana size and spellcasting efficiency. At the moment there's no difference between increasing max mana or fireball damage by 20%. Burn stacking became more meaningful while resist shredding couldn't help you. The Codex gained another layer of strategy and so the Whurgarbl.
Now imagine a special dungeon where one Boss has 50% PR in addition to 25 DR. You'd want to fireball it, except DR is gonna annoy you. And the dungeon also had monsters like this (30% PR and 2DR/level). But there were 2 no xp minibosses who were hard to kill, and one of them droppped a battlemage ring*. You could decide between saving resources or using them to gain spellcasting strenght. Flames and corrosion would be more appealing here too, or as an alternative solution a prepped RBS.
*Or they both dropped a 'FB amulet' = +1 FB damage/level and +1 FB cost.
I like how DR penalizes Monks and any regenfighting where the core is resist stacking, but only slightly affects the high damage based variant. I can see that DR is a less efficient version of resistances against high damage, but the 2 can be combined
Btw here are some monster ideas with DR and similar abilities:
- Gargoyle (140% Attack, 60% HP)
3 DR/level, 15% Phys R, slow strike
The low health makes it ideal for spiking, but spellcasting can barely weaken it, and if you go the melee route you need to be able to tank the hits somehow. Regen fighting is unadvised, but brawling with high health can work out.
- Sewer Slime (70% A, 80% HP)
30% PR, Slimy( has a temporary 6 DR/level called 'Slime', loses 2*your level of it on hits, grows back 1*monster level after each tile of exploration)
The monster has weak stats, but nasty defenses. Powering through with few strong attacks is next to impossible, regular regenfighting is futile. You can get around it with poison, and burning will weaken the Slime regeneration as well. This monster rewards high mana, strategic glyph use, potion races (since each of your new attack becomes significantly stronger), and goblins (with the level up your Slime reducing power grows too).
Because this monster is fairly complex I'd only include it in vicious dungeons.
- Glass Golem (90% A, 80%HP)
1 DR and 1 Damage Reflection/level (suffers this much lower damage while hurting you the same amount)
A simple monster only to introduce flat damage reflection. Hurts you even on dodges, and could kill you when popping your DP. DR or lifesteal can counter it.
Acts as a light version of retaliate FB, it only limits your potential to mix melee with FBs, but pure casters can still will. Annoys B2P users, ignores slowing.