After watching the WWDC Keynote, a few great things stood out to me:

• Unified Alerts and Notifications (like on iOS) which automatically disable when you plug into an external projector – for anyone who does presentations regularly, this resolves the distracting email and IM/Growl alerts that pop up
• Unified Search Field in Safari – Address Bar and Search Box have been combined
• iCloud Tabs – any web pages you have open on any of your other iCloud-connected devices are accessible from your desktop computer
• AirPlay Screen Sharing – so much better than connecting to crappy projectors with dongles

The greatest thing to see in OS X is the bleeding over of iOS features to the desktop environment in ways that make sense. This is the opposite of Microsoft, who’s abruptly moving their users to a completely new environment in Windows 8 (they even killed the Start Menu, even in their ‘classic mode’), Apple is taking little steps, year-by-year to merge the mobile and desktop operating systems.
Slow, careful iterations.
John Gruber pointed this out in a Macworld article he wrote in 2010:

This is how the designers and engineers at Apple roll: They roll.

They take something small, simple, and painstakingly well considered. They ruthlessly cut features to derive the absolute minimum core product they can start with. They polish those features to a shiny intensity. At an anticipated media event, Apple reveals this core product as its Next Big Thing, and explains–no, wait, it simply shows–how painstakingly thoughtful and well designed this core product is. The company releases the product for sale.

Then everyone goes back to Cupertino and rolls. As in, they start with a few tightly packed snowballs and then roll them in more snow to pick up mass until they’ve got a snowman. That’s how Apple builds its platforms. It’s a slow and steady process of continuous iterative improvement–so slow, in fact, that the process is easy to overlook if you’re observing it in real time. Only in hindsight is it obvious just how remarkable Apple’s platform development process is.

Design isn’t just how it looks. It’s also not just a solution to a problem. Design is having a plan.