Jason Snell, in his hands-on of OS X Yosemite:
The design changes in Yosemite are noticeable, but the system is still very recognizably OS X. Longtime Mac users’ fears that the Mac was about to collide with iOS in some kind of crazy peanut butter-chocolate disaster have not been borne out. It’s a forward-thinking design that looks better on Retina displays, pointing the way toward a high-resolution future for Mac users.
I installed the OS on a second partition on my MacBook Air at WWDC, and my thoughts closely track with Snell’s. Yosemite is still OS X. While I don’t care for all the design changes thus far (and am wishing I had a Retina Mac more than ever) it works more or less just like its predecessors. Call me an old man, but that’s what is important to me with these releases.