I've been thinking a bit about Tunnels & Trolls recently. Specifically the "death spiral" that applies to monsters in 5e (I believe it no longer applies in 7e, but as this is being written on my lunch hour, I'll have to verify that later).
Simply put, monsters in T&T do damage based upon their Monster Rating. MR determines the number of d6 they roll for damage, and half of the CURRENT MR is added to the roll as Combat Adds. When a monster takes damage in 5e, it becomes less effective. As damage in T&T is applied to one side or the other (high total minus lower total equals damage to the losing side), as the MR of the monster decreases, it's opportunity to do damage drops quickly. Therefore, "Death Spiral".
Initially, I didn't like the rule, as D&D doesn't have a similar one. It just seemed foreign to my sensibilities.
Now I realize it's damn near genius. Monsters are, for the most part, just there for a single encounter. Rinse and repeat. PCs get whittled down - monsters get beat down.
You see, hit points or health or whatever you want to call it is different for PCs than it is for monsters. PCs need to survive multiple encounters - a monster, one or done.
Minions in 4e come to mind - it's probably the one think introduced with 4e that I like.
Ah well, back to the grindstone...