If you ask an artist why, the greatest artists will tell you, “Well, it was beautiful. It inspired me. It touched me. It reminded me of this or that.”
But you ask a designer why and he says, “Well, I’ve got these 15 different things that all have to coexist in this 800×600 pixel area. And if I do this, that doesn’t work. If I do this, it breaks the other thing. So in order for these three things to be in harmony, I have to do that…”
That points more and more to the challenge to somebody who’s trying to get into or who’s trying to get a job doing UI design, that it’s not about looking at screen shots. Because then you’re putting yourself in the graphic design box.
It’s about your ability to describe problems and your ability to show how it is that a design that you did worked. And if you can show the reasoning and the different relationships between the elements, then you can show that you really know something.