Designer Dmitry Fadeyev covers the basics of UI design. One of his last paragraphs really stands out:
The order the three parts are satisfied is important. A beautiful work that is not usable is worth less than an ugly one that does its function well (unless, of course, if its function is to be beautiful). Thus, we must first of all ensure that every element of the interface is clear, then ensure that their relationships are well defined, and then ensure the work has aesthetic unity.
Fundamentally, core functionality comes before aesthetic beauty. It’s a principle that’s missing from so many UI mockups I see on Dribbble, blogs and other sources.