Starfox
Hero
Area effects and AC. Particularly in 3E and derivates, AC was king. This is still so in 5E, if to a lesser degree with bounded accuracy. If the monsters only hit on a 20, numbers don't matter nearly as much.Indeed!
The deadliness of AD&D combat oftentimes would come down to whether or not there was AoE effects available for the party at the time of the encounter. A "difficult" encounter made by the DM using lots of less-powerful monsters with lower HP could be removed quite easily through the use of a single Sleep spell or heaven-forbid a Fireball a few levels later. [...]
A hearsay anecdote from a campaign I never played in, that feature lots and lots of groups of low-level foes. The fighter-types in the party all had an AC that these monsters need a 20 to hit - except the player relating the anecdote, whose AC was one worse, so the monsters hit on a 19 or 20, thus taking 100% more damage than the rest of the team! They did not use critical hits or had to verify critical hits, not sure which.