I saw a hotel telephone last week that perfectly illustrates one of my favorite themes: simple is not always more usable.
Here's a phone without any buttons. It doesn't get much simpler than that! Yet most people wouldn't even consider it a real telephone, because essential features are missing. How do you choose who you want to call?
If a simple interface is your goal, it's critical to know the essential feature set. Simplify past a certain point, and you risk making your product or system unrecognizable. The best way to know the essential feature set is user research, although a survey of competitive products can substitute in the short term.
But wait, there's more! Over time, the essential feature set increases for a given product. Early phones used to come like this one, with no controls other than a crank to signal an operator to help you. These days, most people won't buy a phone without a display for caller ID, redial and volume controls, and more. Heck, wired phones can seem like a relic compared with the versatility of mobiles.
The ultimate goal is user satisfaction. Meet expectations by delivering the essential feature set, then delight users with a better, more usable interface on those features.