New and improved features are listed here. In general, it's like the upgrading from Windows XP to Windows 7. The main functionalities and UI are same, but got improved everywhere.
My main concerns are:
1. How's the "Add-ins" model get improved?
2. What's going to replace InfoPath?
3. Any AI (Artificial intelligence) in it?
4. Can we put a service (such as "Microsoft SharePoint Foundation Web Application" and "User Profile Synchronization Service") into a Container?
Hopefully we can figure that out in the next six months.
Some screenshots here.
SharePoint Designer 2016 CTP is not released yet, but version 2013 seems still support it, althought the version number is not recognized correctly.