Most recent: Sentinels of the Multiverse, which pointed out the blazingly obvious truth that when your character dies, it sucks, not necessarily because of the death but because you are no longer participating in the game, so give those players something to do.

The First was either AD&D 2nd ed Skill & Powers, which introduced horizontal character growth to my gaming, something that Guild Wars later drove home, or Vampire: The Masquerade for so much (character personalities, PvP, social first, etc.).