Khoi Vinh (via John Gruber):
In many ways, it feels very much like starting over again in the way that Mac OS X’s Aqua interface was a new start, over thirteen years ago. In those nascent stages, Aqua was never particularly beautiful, but it did make a point—it was a radically new kind of interface aesthetic that heralded a new approach to software. And the same ideas that informed later, much more successful iterations of the operating system were clearly present even then. […]
My biggest complaint, personally, is that this fresh coat of paint does a poor job on visual contrast. Interface elements are often so light in color and/or so close to one another in color that they “bleed” into each other all the time. The effect is a blown-out look, as if a novice photographer stepped up the exposure on her camera well beyond advisability.
Just as I wasn’t thrilled with OS X 10.0 when coming from years of OS 8/9, I agree with some of Vinh’s points, although it still works the same and shows great promise. Unlike the early releases of OS X, all my old software works and matches the new aesthetic, and my Macs don’t feel any slower. I do enjoy the more cohesive nature of the interface as a whole and how things do translate from iOS, but might have made a few adjustments if I was doing the design. I played with some of the Accessibility settings during the time I was testing the Yosemite betas—I agree with Gruber that ‘Increase Contrast’ is a neat look that does look like a modern update to the classic Mac OS, although I wish you could cherry-pick some combinations of settings (opaque menu bar with a translucent Dock).