Business
To make something actually easy to use, give each function its own control and group the controls by function. To make it merely look easy, cut the number of controls. You can get both by showing only the controls relevant to the task at hand.
When people fumble with a door, a stove or a shower, the cause is usually a design that ignores how human minds work, not a lack of intelligence. Even very capable people fail with poorly designed objects, so the fix belongs with the object rather than the user.
If a door needs a sign saying "Push" or "Pull," its design has already failed. The shape of the object itself should show what action it allows and where to apply it, so no written instructions are needed.
Using any device means crossing two gaps: working out how to make it do what you want, and then working out from what it shows you whether it did. Design succeeds when it narrows both, by making possible actions plain and making results visible.
Each new feature gives a device more power, but it also makes the device more complex and harder to understand. The trade-off is built in, so designers have to manage it rather than ignore it.
What we need to know can live in our memory or in the environment. Putting it into the world, as a phone does by remembering numbers so we only tap a name, frees people from having to memorize.
Products work when they are built around how people actually think and behave, including quick gut reactions and learned routines, not around how designers wish people behaved. A design is a message from the object to its user, and it has to be written in that user's language.
When an object fails to work as expected, users tend to conclude they are inept rather than that the object gave them no clear guidance. This self-blame hides design faults, because the people who suffer from them don't report them as faults.
Things become easy to use when they are visible, when the layout of the controls naturally matches the layout of what they affect, and when constraints rule out wrong actions. Confusion comes from hidden controls, arbitrary pairings of switch and function, missing feedback, and demands to memorize.
A design that fits our needs well becomes invisible and serves us without drawing attention to itself. Bad design announces its flaws loudly, so we notice failures far more than successes and tend to underrate good work.