Joel Unger approaches the blurry, semi-transparent window aesthetic, recently introduced in iOS7 and now making its way to OS X Yosemite, from the standpoint of evolutionary biology. He argues that the reasons for this latest trend, as well as other trends, are: 1) the visual effect is relatively rare, and 2) the effect is expensive to achieve (in this case expensive in terms of graphics processing power). Both of these go hand in hand given that what is expensive to achieve is probably also going to be rare, at least for the period it still remains expensive. While these two things are initially the differentiator for the trendsetter, they will inevitably me mimicked by others, resulting in a design trend.
Original post by Kerby Rosanes