I watched most of the keynote and the one area that really intrigues me is Continuity
Continuity
Continuity is the biggest new feature of OS X Yosemite, improving the connection between Macs and iOS devices. With Continuity, it's possible to share files between Mac and iOS devices and it's also possible to make phone calls on a Mac via an iPhone.
AirDrop finally allows users to share files, photos, and more between iOS devices and Macs. Previously, this feature only allowed files to be shared from Mac to Mac or from iPhone to iPhone, which means sharing files from an iOS device to a Mac will now be far easier. Apple also debuted Handoff, which is a feature that allows iOS devices and Macs to recognize one another, letting users seamlessly transition from one device to another. For example, if a webpage is opened on Safari on a Mac, a user can pick up an iPhone and continue browsing that same site on his iOS device. In a demo on stage, the function was used to show an email started on the Mac continued on an iPhone.
Through Continuity, iPhone communications integration allows users to make and answer phone calls sent to their iPhones right on their Macs. "Green bubble" SMS messages from Android also now show up on Macs, and it's easier than ever for a Mac to connect to an iPhone hotspot.