Single mechanics from an RPG you love

[J.M]

Most innovative mechanic that actually works for the game: the Jenga tower in Dread.

Favorite "new" mechanic that's gone mainstream: degrees of success/failure ("but...")
It brings something fun to the table compared to D&D/OSR games. I personally like it as long as it's implemented simply and in moderation (too burdensome if you have to come up with a multi-dimensional outcome on every roll).

 

[Enevhar Aldarion]

The Take 10/Take 20 system in 3.x D&D. Anything that helps limit the number of dice being rolled in non-combat situations is good. It is too bad this did not translate better into the Passive checks in 5E.

Even though I would never use them now, the Critical Hit and Miss charts from Rolemaster were crazy and fun.

 

[The Crimson Binome]

I want to say that it was an earlier edition of Shadowrun, which means it's probably also in White Wolf since that's where they stole most of their rules from, but I like the idea of opposed melee attack rolls.

Instead of you attacking and them defending (like in most games), attacking someone in melee meant that they rolled their own weapon skill against yours, and whichever side scored better was the one who hit. If your enemy is some sword saint with a magic katana, and you swing at them with a baseball bat, then there's a very good chance that your attack will result in them cutting your head off.

Of course, the reason this rule works is that the PCs are much more likely to be the overpowered super ninja, so this is a great way of running through a bunch of chumps quickly.

 

[Gradine]

I really enjoyed Wounds/Vitality from Star Wars d20, and we ported it over to several 3.5 campaigns. It's unnecessarily fiddly (with the DR on armor and class defense) and I don't think I'd ever use it again, but I enjoyed it for what it was when I used it.

 

[Mercule]

Aspects (and invoke/compel) from Fate.