What are some of the unique game mechanics you loved in individual games/series?

I loved the addition of Paragon/Renegade actions in Mass Effect 2&3.

Yeah there was the regular morality system, unlocking conversation options based on how many points you had.

But this gave you a way to interrupt the flow of conversation amd events with a nice or hadass action, regardless of what morality path you were pursuing. It was a nice way to add more complexity to your character beyond all nice/naughty.