what game mechanic totally blew your mind?

Just curious. What game mechanic totally blew your mind when you first played with it or learned about it? Even if that was a long time ago.

The first time I played Ark Nova and I had the same 5 actions - but they would have different powers based on my timing... I knew I was participating in something special. I'm not aware of any other games that have that same mechanic... but I'd be happy to learn about them.

What's on your list?