OK, It is hard to remember the early days, but I will give it a try
BECMI: Immortal / epic rules. Rules for playing immortals that only tangentially remind you of the rest of the game.
1: the DMG, PHB, MM division of books
2: didn't play this edition, but I will go with multitude of discreet settings with unique mechanics
3: 3 save system
4: this one is hard, as there is a lot to like but I will go with monster design (kind cheating as this includes lots of things, but I am sticking to it)
5: this one is also hard, but I think I will go with disadvantage / advantage