Nick Keppol at MartianCraft’s blog:
I want to focus on my favorite visual update in Yosemite — the dock icons. Before Yosemite, Apple maintained a system for icon design through a checklist of mostly unstated and understood guidelines paired with a few specific recommendations in the Human Interface Guidelines (HIG). With Yosemite, that system becomes more consistent, and regular, yet the HIG remains silent on the specifics.
This is a really good, in-depth look at Apple’s new icons and how they relate to the overall new design in OS X Yosemite.