The most obvious change, visually at least, is the new interface. Yosemite does to the Mac what iOS 7 did to the iPhone and iPad. Its user interface is flatter – though not flat, there are still drop shadows and other nods to the third dimension, it's just that now they exist for a purpose rather than being merely eye candy. No more glassy textures.
Not a huge amount has changed here, but there is one key addition: iCloud Drive. Your iCloud storage drive now shows up in the Finder and you can drag and drop files and folders to it just like any other location. It also displays the files you've opted to store there from apps like Pages, Numbers, and Text Edit.
Brand new in Yosemite is Dark Mode, which turns some aspects of the OS a much darker shade of grey, to make it more comfortable to use your Mac in dim lighting. These include the Finder menu bar, Dock, and application switcher. During the beta period some elements of Dark Mode, such as Finder menus, were poorly implemented, and it remains to be seen whether they have been fixed in time for the full release.
The manipulation of my work go faster and more efficiently.