[Question] Unique game mechanics -- The best and the worst

OP's post over in the BioShock thread about original game mechanics got me thinking: How does the game industry manage to recycle the same mechanics over and over again with shinier packaging? The only answer I can quickly think of is that they do it by combining them or using some minor gimmick as a reason to get us to shoot more bad guys.

But ranting aside, I can distinctly recall some very unique game mechanics that I've never seen before. Persona 3 having something very close to a dating-sim game affecting your performance in the included RPG portion. Katamari Damashii's "roll shit up" mechanic. Those are ones that worked well, and help make the game great.

However, Final Fantasy 2's "You hit something, your strength goes up" mechanic was completely broken. Not only was the cause and effect occasionally bonkers, but you could even hit yourself to raise your stats!

My question to GFF is what are the best and the worst UNIQUE game mechanics you've seen? By UNIQUE, I mean it's confined either to one game or one series, with MAYBE an appearance in another game close to the original. Nothing like "Back in the day, FF had experience points!" That doesn't count, just because it was unique the first time doesnt mean it's still unique 50,000 applications later.

Jam it back in, in the dark.