The biggest mental shift is how in 2e lower defenses are better than higher ones, a completely unintuitive system that they dropped for later editions. Everything else is, while not the same, at least similar enough that you wouldn’t get confused.

The lack of cantrips is another, making low level casters miserable to play due to how carefully you need to ration their handful of spells. The lack of bounded accuracy and weapon proficiency system means they won’t be any use in melee, so they’ll mainly spend their time in combat dealing a few points of damage from a distance using weak slings.

(Though higher-level casters are absurd and can basically carry your party, as is tradition for old-school tabletop RPGs.)