It does not make a product more innovative, powerful or valuable than it really is. It does not attempt to manipulate the consumer with promises that cannot be kept.
Honest design is tricky to define. Dieter Rams doesn’t elaborate, his designs are explained by themselves. Nowadays design tends to overdesign in order to make it “innovative,” “interesting” or “appealing” to consumers — yes, I’ve assumed a sarcastic tone… — Most people think that more worth more, that it has more value and this is way far from the truth.
So, What does “honest” mean?
It is what you think it is. Its form and materials indicate its nature. Well-defined affordances indicate functionality clearly.
E.g. metal should feel like metal and wood should feel like wood. Chrome is a fine finish for a metal bumper; plastic utensils with a chrome finish are a lie.
It does what you think it does. Interactions are as much a part of honesty as materials are. Something that looks like a dial or button should turn or depress, and you should understand the effect of that action.
Honest doesn’t mean minimalist. That is too simplistic. It is naive. Honest, means not to lie when it comes to design.
Take Apple’s iPad; it feels like a solid element, like there’s no machinery inside. Apple’s product design has become unibody to belie the complexity within. Because humans cannot perceive circuitry in operation. The whole experience has to support the design story for it to be believable.
That’s what honest design is — an object that you perceive as having a consistent model of interaction.
Dieter Rams: Ten Principles for Good Design – Readymag.