Cool user interfaces have The Flow. They let you do what you need to do without consideration or thought. bad user interfaces break The Flow. What do I mean? Let me explain by example.
This is my digital camera. More specifically it is the battery compartment of my digital camera:
The battery fits into this camera in exactly one way. Turn it upside down or around, and it doesn’t fit. I invariably try to shove the battery in backwards. Even when I stop and think about the problem and realize that my natural instinct of which way the battery goes is wrong, and deliberately reverse it, I still put it in the wrong way! I have no idea how Panasonic managed this, but they did. This device does not have The Flow.