Designer Jessica Enders, writing for A List Apart:
The problem is that in the push for simplicity, flat UIs may have gone too far. With content, things like drop shadows, gradients, and borders may well be no more than useless “embellishments.” When we read a multi-page news article, it doesn’t matter much whether the mechanism to move to the next page is a button or a link. With forms, however, distinguishing between a button and a link matters far more.
A thesis that argues for more visual contrast than average for forms may sound a bit simplistic. But it’s not; Jessica goes into some really excellent design examples to show how just a tiny bit more distinction or hierarchy can have significant form conversion effects.