On to the belated day 11: flavour.
With passing years my preferences changed drastically. Like many, my early games were AD&D 2nd Edition, Cyberpunk 2013/2020, Vampire: the Masquerade … As time passed I stopped caring about what games like that brought to the table. I likened a bunch of crunchy things to a wallpaper – a decoration, not a thing that mattered. I do not look for mechanics that reinforce system mastery; what I care is the mechanics of a game that bring flavour to my table.
It can be a simple thing like the wording of the thing like the basic moves in Apocalypse World. Just saying them out loud feels like honey in my mouth – go aggro, or read a sitch, or do something under fire … you can taste those phrases, they put you into the game.
Or the base of Fate where the things you roll is the thing the game wants you to care about – skills, approaches, careers … It is still a simulationist approach, but the thing we are codifying as important is the thing that is important to the game, not to the real world physics.
A single mechanic from a game so vital it defines the game, even elevates it. I’m looking at you, wonderful hunger dice of Vampire’s fifth edition.