From OD&D:
4 Classes instead of 1, synergistic enough to allow for cooperative play, if desired. The first RPG and it supported four roles, not one.
Hit Points
Races, Still roleplaying, but included variations on being "human" (i.e. "demi-human")
Saving Throws
Vancian Spell slots
Morale
"Coin"-based encumbrance
"Dungeon" levels as Difficulty levels
"Character" levels as Player ability levels
Interdependent play due to Lawful and Neutral (unaligned or otherwise) Alignments
DM Screens, to keep the rules secret
Dungeons, all types of spatial exploration
"Magic" Items, for all imaginable types of object exploration
Traps, mixing dungeons and magic explorations together
"Monsters" as anything non-PC, allowing NPCs/ auxiliary personas near unlimited variability
Referees, leaving the consequences of every action up to neither the Players nor an arbitrary judge figure