We first learned of Microsoft's Xbox One Windows 10 plans in advance of E3, and while Microsoft hasn't gone in-depth about all of what the update will bring to the table, there's enough meat here to gain a basic understanding.
It's no doubt obvious, but Windows 10 on Xbox One isn't going to look like Windows 10 on the PC. Like the mobile variant of the OS, Xbox One will be treated to a finely-tuned Windows 10 that's optimized for performance and ease-of-use. Cortana will be a major addition, allowing users to control various Xbox One functions with their voice (something that will require the Kinect sensor).
What Windows 10 on Xbox One will no doubt tout is the fact that it works in perfect unison with the desktop OS. It already has the ability to stream back and forth between a PC, so it seems likely that this major update will help refine that feature even further.
While we still don't know everything there is to know, it does seem that this is one update worth getting excited over.