I had very little trouble figuring out what my answer would be for this one. As soon as I looked at the question, I knew what it would be. My choice for my favorite game mechanic that I feel is pretty revolutionary is Dread's action resolution system.
Dread is an horror roleplaying game published by The Impossible Dream. The players are handed sheets possessing nearly a dozen questions they must answer to create their characters. Most of these questions are loaded ones, and the only rule you must follow when answering them is to not contradict the question with your answer. There are no skills or talents within the game, so characters are represented purely through roleplay.
With that in mind, you might be asking yourself, "How do you resolve actions within the game?" That's pretty simple: You use a Jenga tower.
Whenever a player attempts an action where failure has dire consequences associated with it, they must pull a block from the tower. They can only use one hand and the pulled block must be placed atop the tower. The action succeeds if the tower remains standing after the block has been placed. However, the action fails if the tower collapses and the character is removed from the story.
The reason why I love this mechanic so much is that it models the suspense and tension of a horror tale perfectly. Each time a block is removed from the tower, it gets less and less stable. You know the tower will eventually collapse, but you don't know when. This makes each pull incredibly tense, wondering if you will be the one to knock it over, or will you succeed and live to make another pull.
When the tower falls, it represents that drop in tension that happens within a good horror tale, giving you a moment of relief before it is rebuilt and the game continues, the tension building once again. This is the reason why Dread is one of my favorite horror games and my go to game for one shot sessions.