Preview at WWDC likely in June, followed by September launch.
Apple Releases OS X Yosemite Developer Preview 3
The update, build 14A283o, can be downloaded from the Mac App Store or the Mac Dev Center.
OS X Yosemite brings a flatter, more modern look to OS X, with an emphasis on translucency and redesigned dock, windows, and more. It also includes a multitude of new features, such as improved integration with iOS 8 through Continuity, a new "Today" view in Notification Center that offers integration with third-party apps, a retooled Spotlight search with new data sources, and several new features for apps like Mail, Safari, and Messages.
Ahead of today's new beta release, Apple announced plans to wipe all CloudKit data for the iOS and Yosemite betas, including data from iCloud Drive, iCloud Photo Library, and Mail Drop.
OS X Yosemite is currently only available to developers, but Apple plans to offer a beta version of the software to Mac users at some point during the summer. A public release of OS X Yosemite is expected in the fall, after several additional beta iterations.
Dark Mode - First previewed during WWDC, Dark Mode is now officially available as of the third developer preview of Yosemite. Previously, Dark Mode could only be enabled via terminal command.
Interface tweaks - System Preferences, Mail, and iCloud Preferences have all seen some interface tweaks in DP3 as noted by 9to5Mac. There are also larger App Icons for Notifications and a new font in the Notification Center/Safari Favorites Bar.
QuickTime - QuickTime has an updated icon.
Settings - Apple appears to have removed the Continuity/AirDrop settings from the System Report in Developer Preview 3. Previously, these settings let users know if Continuity and AirDrop were accessible on their machines. Apple also removed an "Allow Handoff between this Mac and Your iCloud devices" toggle from System Preferences on some machines.