One surprise

Every design should have at least one moment of surprise.

No matter how austere, no matter how formalist, no matter how devoted to function, your design should have a moment of delight. That's how you know it was designed for humans.

Polish, polish, polish, but be careful not to scrub away the moments that have no purpose other than to delight.

Examples:

• The rubber band effect in the original iPhone

Also: design patterns.