Look at Xcode 5 for the future of OS X design
Many here have been speculating what OS X will look like if it gets a redesign. I think the best indication of this is what we see in Xcode 5. It is the only one of Apple's new OS X apps that have not conformed to their traditional OS X design.
For reference, here is Xcode 4:
And the new Xcode 5:
- Toolbar buttons will lose their borders, in favour of buttons that change colour when activated
- Slightly flatter. Specifically take a look at the sidebar, and status area.
- Whiter. Unnecessary background colours will be removed.
- Switches are coming to OS X. Perhaps flatter icons as well (probably not).
That's about it. It will be a welcome refresh, but nothing world changing like in iOS 7.