There are several major changes I've seen discussed:
1) Container support. With containers all the rage, Microsoft is building it into the OS. There will also be a number of enhanced cloud features like improved OneDrive, SharePoint integration and other tweaks.
2) SharePoint enhancements, including a new way of patching.
3) Improved Hyper-V, including server resilliancy and a new micro-server that loads the bare minimum of the OS needed to deploy extremely small footprint servers.
4) Storage enhancements that include VM storage performance, support for Storag Spaces across multiple servers and synchronous replication.
5) New security and management features for tighter management of the server and intrusion detection.