Perhaps the most obvious addition (after upgrading Apple TV) is the addition of Siri to the new Macs, which will make Search much easier than before.
Copy on your computer, paste to your phone. The multi-device clipboard can erase the need to email oneself or use Dropbox in certain cases.
As a security upgrade, simply having your phone near your laptop will create an automatic unlock feature to use each device.
Redundant File Block
The new Mac can actually determine which files are redundancies and erase them from your data to save space.
More details available at Popular Science.