Shemeska
Adventurer
I' in the position of actually playing 4th Edition with people like that (well, 18 years instead of 20, but close enough). They spotted the difference. We swapped into a 3rd Edition game. They commented on the fact that THAC0 was gone, but the rest of the game played the same for them.
It's less the rules necessary I had in mind than changes to flavor elements like spells largely going all combat with any noncombat effects relegated to rituals in 4e, archons going from being Lawful Good celestials to evil elemental monsters, the classic color and element archetype of chromatic and metallic dragons being upended, succubi being devils now, half of the alignments vanishing, etc. Mechanics change and mean little to me, but those sort of flavor elements that held for decades suddenly being redefined would throw me for a loop if I suddenly encountered them.