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.
—Nick Keppol doing an awesome job inspecting the icons in OS X Yosemite