Donald Norman’s key principles of design:

1. Make it easy to determine what actions are possible at a particular moment.
2. Make things visible – the conceptual model of the system, the alternative actions and the results of actions.
3. Make it easy to evaluate the current state of the system.
4. Follow natural mappings between intentions and the required actions; between actions and the resulting effect; and between the information that is visible and the interpretation of the system state.

In other words, the user should be able to figure out what to do without much instruction. He should also be able to tell what is going on. The reason for the design should be a simple explanation, even if that is required. If the explanation leaves the user confused, the design has failed. The interesting observation that Don Norman makes is this: when we are not able to figure out how to make a call from the latest smartphone or cannot locate a simple formatting command in a word processor, it is not because we are dumb, it is just another case of bad product design.