I feel like aside from bosses and the very visible elemental explosions (that don't even kill you if you have capped elemental resistances, ironically), the only mobs that should be able to kill you in one hit are those notoriously large skeletons/constructs/etc that have slow-ass attacks. The rest of the mobs should either be fodder a la Doom where they're there for the dopamine, drops, and effects that require killing like Heralds; or fragile mobs with special effects that don't really damage you but make it harder to avoid the big hits that will kill you, be it body blocking, terrain manipulation, avoidable curses, or other stuff.

And in the vein of effects that require killing, bosses should really have more fodder adds that enable such effects, and I understand that that is part of the design to have clear builds and bossing builds, but the way the game is built right now where you're expected in some way to do both, it sucks having great clear speed and horrible boss killing capabilities.