Favourite Revolutionary Game Mechanic
Linguistically, a "game mechanic" would be a person who manipulates game mechanisms... On the same way that an "automobile mechanic" would be a person who manipulates the mechanisms in automobiles.
Who do I consider the revolutionary game mechanics? Probably +luke crane, +Vincent Baker, +John Harper, I don't know that +Monte Cook has been particularly revolutionary lately, there's probably more of a cult of personality thing going on there...but he did do some great stuff years (decades) ago.
I deliberately avoided stating my favourite writer in an earlier question, and I'm not going to get into it here.
In a more colloquial sense, a "game mechanic" has come to mean a "game mechanism". Revolution implies a cycle.
Is there a game mechanism that has simply brought things full circle?
Arguably, the Apocalypse Engine does this. It pulled a lot of indie game concepts back into the old style of gaming, and swept up a big part of the indie movement. I've got a love-hate relationship with the Apocalypse Engine, I love the fact that it does certain things well... I hate the fact that hundreds of lazy game designers have hacked it into (what seems like) every second game release these days.
I much preferred "otherkind dice" as a interesting system for generating situational outcomes, but this generally now seems to be considered a step toward the Apocalypse Engine. It's not cyclical though, it was a step in a very new direction...much like the notion of diceless gaming that Eric Wujick released onto the world in Amber.
If we're talking about my favorite game mechanism that pushed gaming in a new direction, I'd probably go with that last example... Dicelessness in Amber.