On my 2012 MacBook Pro with Retina display, installation took about a half hour; it took nearly an hour on my late-2012 Mac Mini (which does not have an SSD).
A Mac interface with iOS flourishes
The design focus in Yosemite, according to Apple, was to bring better focus on content. The designers started by toning down window ornamentation (even consolidating and streamlining the window toolbars in Safari and other system apps), and flattening buttons and other interface elements.
One of the more noticeable changes is in the system-wide font: Apple is now using a thinner font for menus and labels. In addition, the Dock is no longer faux-3D but has reverted to a flat design last seen in OS X 10.4. System and app icons have received the flat makeover first seen in iOS 7. And the stoplight controls located at the top-left of every OS X window no longer sport any element of depth; they are just red, mustard and green circles -- and the green button now toggles the app to a full-screen view.
The Yosemite interface adopts design cues from iOS 7/8; specifically, some interface elements allow color, but not much detail, of the content beneath to show through. The sidebars of apps and Finder windows, for instance, display the diffused colors of the content beneath them, and app toolbars display the colors of the in-app content underneath. This adds a personalized splash of color to what is otherwise a gray-themed system.
Apple engineers have also included a new Dark Mode, which can be activated by toggling on System Preferences > General > Use dark menu and Dock. Doing so changes the menu from translucent white to translucent black, while text and menu widgets are displayed as white on a black background. The Dock changes its background to a darker shade, too, but that's it: this mode doesn't change any other interface elements. Overall, the system trimmings remain light gray. I (I expect that many third-party menu widgets will need to be updated, as most look pretty terrible at the moment in this mode.)
Yosemite's biggest update, Continuity, is a set of features that use Wi-Fi and Bluetooth to tie Mac and iOS devices together.
A few notes: Be aware that your devices must be signed into your iCloud account for most of these features to work and that your iDevices must be running iOS 8. In addition, remember this is beta software -- some users have reported problems.
Handoff: If you have more than one Apple device -- say, a Mac and an iPhone and/or iPad -- Handoff will probably be your favorite feature; it's definitely mine. Basically, Handoff lets any device take over your current task.
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