The best user interfaces are the ones that need no documentation. They are intuitive, and by the very act of using them, they educate you to as their purpose. They tell you about how to efficiently interact with them to reap the most value. There are, of course, many concerns that go into making such an interface, but foremost amongst the key ingredients is reactivity. That is, the principle that any changes to application state are reflected immediately to the user, visually or otherwise.