This one is really hard. I am having a hard time coming up with a mechanic that I feel is really revolutionary. And I am going to answer this from the perspective of what felt revolutionary to me when I encountered it. That means that if someone else did it first, but I never encountered it, too bad.
While there are plenty of times that I have found a mechanic clever, I can only think of two times that I can say I was surprised by one. The first was saving throws, it actually took a while for me to get my head around those. Part of that was that I was 10 years old, the other part o it was that it was the opposite of the way everything else in the game worked. You generally rolled to make stuff happen to another character, not to keep it from happening to you. I was comfortable with hit points when I first encountered them, they were the same as hearts in Zelda, but rolling to keep a status from being applied to my character was a bit mind bending.
The other was the conflict resolution system used in Dogs in the Vineyard. This was really the first time I ever encountered a game that was not just "D&D but different". Prior to Vince explaining it to me, I thought I had encountered games that were more than just "D&D but different", but I realized that was not true immediately afterwards. It was the first time that I encountered a game where I could say, "This is what RPGs might look like if the person who invented them was not coming from wargames." There have been other games since that clearly not drawn from the D&D tradition, Dread is what RPGs would look like if the inventors came from Jenga, but DitV was the first I encountered.
Dogs in the Vineyard's core mechanic is my favorite, and possibly only, true revolutionary mechanic I have encountered.
2 days ago