Finder Icon
Watch out for the Finder icon. It seemed to show a bit goofy during the keynote but online at the website it appears as just a more saturated version of the current icon? But wait my eyes don't deceive me, on the page for xCode 6 the Goofy Finder interloper appears.
https://developer.apple.com/xcode/
Overall it looks a major change in the UI and time will tell. IOS 7 was a change from earlier versions and on close inspection introduced some very interesting and powerful UX features. I feel the transparency should be toned down a bit in areas where underlapping layers distort key areas of content, especially where text needs to be read.
Even in Lion there are instances where this effects legibility: sometimes on modal sheets and the pop-up dictionary definitions. In fact some of the dynamic text/Text kit technology of IOS could be but to good use in Mac OS X, and hopefully this will be the case with Yosemite.
I think a little bit of desaturation on some of the UI elements, and the Finder folder icons need a bit more work. Overall a slight toning back to the look of Mavericks should do it.
Great thing that there is a Beta program inviting feedback, so depending on you beta testers to give good feedback. I my self will not be participating, because current conditions keep me on 10.7.5 on my trusty Mac Pro 1.1.
Revert to traditional Finder Icon please. We don't want the Windows users having a laugh or I will have to hide my future Macs from their mirth and that will be lost opportunity for Apple to advertise their wares to potential new users. Pull back a bit from the Fisher Price look, maintain some of the gravitas of the more recent OS X releases (Lion/Mavericks) sans the Skeumorphism.