In 2002 Jef Raskin wrote to Tom Gilb "A nearly one page summary of design rules".

The first principle. When using a product to help you do a task, the product should only help and never distract you from the task.
The second principle: An interface should be reliable.
The third princple: An interface should be efficient and as simple as possible.
The fourth principle: The suitability of an interface can only be determined by testing.
The fifth principle: An interface should be pleasant in tone and visually attractive.

Then Jef Raskin concludes:

An interface should be effective, habituating, reliable, efficient, and tested. To the extend that doing so does not conflict with these essentials, an interface should also be attractive.

[read Jef’s nearly one page summary of Raskin’s design rules]