Image credit: Instructions taken from Peeron (specifically).
Okay, once you get rid of the rules and put them into the design and the interactions around the design, two things happen.
The first is that the discovery of uses can’t happen through the rulebook, and it happens through play instead.
Lego is the canonical example of play. You don’t know, ahead of time, all the millions of things you can do with Lego pieces, but by simply looking at them you can tell what you can do next. The studs and holes tell you that. The solid plastic tells you that the bricks aren’t going to change in size and shape.
With that security, play becomes safe and enjoyable. You play to find the possibilities of a system. Some software works really well like this. Most doesn’t.