Probably Aspects in Fate, although (as has been mentioned by others) they're hardly a new mechanic at this point. Aspects, if you haven't been paying attention to the goodness that is Fate, are a way of drawing attention to something in the game world that is significant in the fiction. Characters have Aspects to support important elements of their character and make them relevant in gameplay. A character's Weird Intuition or Cultish Affection for Libraries could be an important part of a Fate game, just by assigning an Aspect to them.
Ditto for pieces of equipment, or elements of the scene, or even moves that the characters make. If it's important, it can be an Aspect. Not everything can be an Aspect, because a lot of things aren't important - that's a rookie Fate mistake. But Aspects are such a powerful, broad tool to reframe the conversation at the game table that they changed the way I play and the way I think about games.
More recently, I've gotten a lot of good gameplay out of the dice mechanics in APOCALYPSE WORLD and all its many gaming offspring. Formalizing certain actions as "Moves" that must be resolved by a dice mechanic, and not by the whim of the GM, and then framing the resolution of those moves as either a) Success and Then Some, b) Success with a Catch, or c) Failure and the GM Gets To Do Something Nasty - that's a powerful way of talking about player character behaviour. It says what's important enough to demand mechanical weight in a non-negotiable manner, and ensures that the mechanics always give you an interesting result. Your end product is never going to be "I take an action that's not that important and nothing interesting results from it."
Like Aspects, that way of thinking about mechanics changes the way you think about gaming as a whole.
26. What's your favourite gaming inspiration?
Probably research I do in preparation for a game. Often, I'll do some reading and discover things about the topic that I never suspected, or an angle on it that I wasn't prepared for. It reframes my thoughts, and makes me look further and read more. This can be a hazard, when you're prepping for something specific, but it almost always makes your games richer and more complex.
Reading The Ghost Map started me thinking about Victorian England in a different way, picturing the world at street level, and most of all trying to wrap my brain around the Victorian mindset. All of the incidental research I did for SEVEN STARS OF ATLANTIS helped me bring the Dirty Thirties of the pulps to life, and gave me some amazing incidental details to lend my story depth. A casual search for a British airbase where an aviator might store a dirigible let me discover an experimental flying wing program. Reading about Shanghai introduced me to the Russian immigrant population there, and set the stage for one of the most important story elements - the inclusion of (not assassinated, or rather not successfully assassinated) Anastasia Romanov and her undead guardian Rasputin in the story, kept alive by the power of one of the Seven Stars.
Research almost always makes things better, even if it's just insprirational reading / viewing / listening. I recommend it.