Never present a power-user option in such a way that normal users must learn all about it in order to know they don't need to use it.

If you are demanding registration before checkout, you need to cease this practice immediately. It is costing you a fortune.

Constrain the user's expectations to match the abilities of the software.

Never present a power-user option in such a way that normal users must learn all about it in order to know they don't need to use it.

The only intuitive interface is the nipple. Everything else is learned.

It's hard to read through a book on the principles of magic without glancing at the cover periodically to make sure it isn't a book on software design.

As for the Sun mouse, I'm not a big multi-button mouse fan, because I just can't remember which button to push when. I rather like the Macintosh system of using four modifier keys with the mouse.