What is your favorite mechanic from the last game you played? Preferably a table top RPG, but if you played some other game with a mechanic you want to gush about, than go for it.
For me, the last game I played was FATE: Core. The mechanic I love the most about it is the Tagging system for Aspects.
For those that don't know, an Aspect is a word or short phrase that describes a character. Something like "The #1 Sword in all Bravosi" for a certain Water Dancer from Game of Thrones, or say "Leader of the Autobots" if we were making a sheet for Optimus Prime. Now, a player can invoke their aspect to give a bonus to themselves on a roll relevant to the aspect. However, the GM - or other players - can tag the aspect to force the character into a bad situation or certain course of action based on the aspect.
For example, say there is someone loudly proclaiming to be the greatest swordsman in all the world. The person is a drunk and a fool, but your character needs a distraction to slip past some guards unnoticed. You give a Fate Point to the player and say you are tagging his aspect "#1 Sword in All Bravosi" in regards to the drunk. The player then has their character go up and dispute the drunk's claim, a duel ensues, and you have your window.
I love this mechanic for a number of reasons. One, is that it pays the player every time a negative part of who their character is comes up, which encourages players to play into their flaws/negatives to create interesting situations. Two, is that it lets the GM and other players bring another character into a scene or the spotlight for a moment or two.
With this mechanic as a GM I've paid PCs to all be present for a big thing. As a player I've paid a PC to have them come along with the group. I've even used it to pay a PC to take charge and grab what became a crowning moment of awesome.
It's a wonderful mechanic, and I wish more games had something like it. So, what is yours?