"Everything shall be simpler, but not simple." — Albert Einstein
1. Be useful. (1.5 times more important as usability.) "It doesn't matter how useable or desirable it is if it doesn't serve some purpose in your user's life.
2. Be usable. "People buy products for 2 reasons: easing pain and providing pleasure."
3. Be enjoyable. "Passive magic, when everything feels so intuitive and effortless."
Read more at Co. Design.