Favourite Revolutionary Game Mechanic
This: “Every moment of play, roll dice or say yes.”
I think that this phrase from Dogs In The Vineyard by Vincent Baker has more functional GM practice packed into its five words than almost any GM guide ever published for any game ever — which is probably why the adulterated version (“Say yes or roll the dice”) has appeared in many RPGs published since.
This is the phrase that tells the GM to be a fan of the players. This is the phrase that tells us that system matters. This is the phrase that tells us to play to find out what happens. This is the phrase that tells us to focus on what’s important and move the game forward. This is the phrase that fixed roleplaying.