I love the approach from Gary Simon on this video highlighting the pillars of good front end design. He puts forward the following key elements of UI design:
Colour and contrast, Whitespace, Alignment, Scale, Visual Hierachy, Consistency, Simplicity and Typography.
While I believe Gary has hit the nail on the head with these categories, I feel he missed touching on one key element.
The missing element that I believe a lot of developers fail to consider is Accessibility. It is estimated that one fifth of the population is suffering from some form of disability, this is a huge market, and another filter through which you should be designing your pages.