Clarity is the first and most important job of any interface. To be effective using an interface you've designed, people must be able to recognize what it is, care about why they would use it, understand what the interface is helping them interact with, predict what will happen when they use it, and then successfully interact with it.
Interfaces exist to enable interaction between humans and our world. They can help clarify, illuminate, enable, show relationships, bring us together, pull us apart, manage our expectations, and give us access to services.
We live in a world of interruption. It's hard to read in peace anymore without something trying to distract us and direct our attention elsewhere. Attention is precious.
Humans are most comfortable when they feel in control of themselves and their environment. Thoughtless software takes away that comfort by forcing people into unplanned interactions, confusing pathways, and surprising outcomes.
The best interface is none at all, when we are able to directly manipulate the physical objects in our world. Since this is not always possible, and objects are increasingly informational, we create interfaces to help us interact with them.
Every screen we design should support a single action of real value to the person using it. This makes it easier to learn, easier to use, and easier to add to or build on when necessary.
Screens with a single primary action can have multiple secondary actions but they need to be kept secondary!
As in most design disciplines, interface design is successful when people are using what you've designed.