OS X 10.10 is internally codenamed Syrah (yes, another wine), and sources hint that Apple has been toying with a new design across the system that is akin, but not as dramatically different, to the new designs found in iOS 7 and iCloud.com.
I hadn’t heard the codename, but I did hear a whisper or two that work had begun on the next iteration on OS X with an emphasis being on re-unifying the look & feel of OS X and iOS. Rene Ritchie has apparently been hearing the same as well.
There’s no question that OS X now looks dated in our new iOS 7 world. What’s weird is that the just-reached-GM OS X Mavericks only has the smallest design updates (Calendar loses the faux leather, for example) even though it’s coming after iOS 7. But that’s the harsh world of limited resources. Even Apple doesn’t have unlimited designers.
Also interesting: the notion that this would be OS X 10.10 (as traffic logs indicate) and not OS XI.