It really all depends. In general, I loved Super Mario RPG for its unique take on the Mario world, fleshing out its own little part of the Mushroom Kingdom that just hasn't been beaten by any other as of yet simply because it looked, felt, and played differently than any other Mario or RPG of its time.
So, for me, it's how well you take an already established setting/genre and flip it around without completely alienating me. [sub]Hell, that's one of the reasons Hyrule Warriors has me a little giddy, and that's not even an RPG.[/sub] Then again, I suppose that's not a mechanic as much as it is just an excuse for me to talk about Super Mario RPG.
Alright: For me it's the little things you can do after you've played the game enough. Dark Souls in particular has a ton of these, from getting the Demon's Hammer in the tutorial boss fight by beating it the first time you encounter it, to skipping an entire section of the game because you got the Master Key.