I read an article this week that really stuck with me, an article about the ten principles of good design by Vitsoe's designer Dieter Rams. Rams set out to distinguish the most important principles of good design, after becoming increasing concerned with the abundance of "bad" design (quotations my own) he observed in the world.
I would encourage anyone to read the full article, but the two principles I'd like to highlight here are, number two, "Good design makes a product useful", and number four, "Good design makes a product understandable". These two principles drive each other and go hand in hand with the objective of the design: if your product cannot be understood, it won't be used; if your product cannot be used, it must not be understandable. I am not sure if it can be stated any more simply: design with your user in mind, and testtesttesttest.