This year Stephen Hackett’s written an excellent review of Yosemite’s design:
At this point, I’m not sure referring to Yosemite’s UI as Aqua is even correct. If Aqua defines the structures and underlying philosophies that shape OS X, then it’s still present, despite the ever-growing number of changes from those original lickable buttons. However, if Aqua is just a collection of colorful buttons, windows with title bars and a predictable color scheme, it may have died the second Craig Federighi showed off Yosemite this summer.
Aqua is gone. We lick the interface no more.
—Linked by Jason Snell