And why? What makes it good for sandboxing?
I've been pondering a lot about this lately, reward cycles and whatnot. D&D for example has the advantage for bribing you to seek treasure and kill monsters (depending on the edition), but what else can strengthen the structure of an "open-world" Step on Up game? Like, "here's your playground, full of challenges I wonder what you're going to do about." What games already support that out of the box, in your opinion, and what could be done better.
Be warned: I probably won't end up running anything you suggest, I just want to see what's on the table.