To make people happy, you have to let them feel like they are in control of their environment. To do this, you need to correctly interpret their actions. The interface needs to behave in the way they are expecting it to behave.
Thus, the cardinal axiom of all user interface design:
A user interface is well-designed when the program behaves exactly how the user thought it would.
As Hillel said, everything else is commentary. All the other rules of good UI design are just corollaries.
| Author | Joel Spolsky |
| Work | User Interface Design for Programmers - Chapter 1 |