Media event set for September 9, launch likely ~10 days later.
OS X 10.10 to Feature 'Flatter' Look, but Not as Stark as iOS 7
Instead [of a complete iOS 7-like overhaul], I am expecting OS X 10.10 to have user-interface tweaks that will make the interface “flatter,” but not as stark as iOS 7′s look. I’d also expect some blur and translucency effects in a few places, but not in anyway that is central to the experience like it is on iOS. There will be a little bit more white space, more defined menu bars, and squared-off window controls, but I would not expect a full color palette change and redesigns for every single application and icon.The report indicates that internal build numbers for OS X 10.10 are currently around 14Z109 and that new features could include improved notifications, cross-platform AirDrop support to and from iOS 8, and possible Siri integration.
MacRumors has been seeing fairly low levels of activity coming from machines running OS X 10.10 from Apple's network, and that activity has unsurprisingly been beginning to ramp up this month following the holiday break. With Apple having accelerated its OS X release cycles over the past several major updates, a public release for OS X 10.10 could come late this year or early next year.