It will address bugs in older versions and make attempts to run faster. It will have an optional Windows Linux System to run Linux shells under Windows 10. It will capture more data and send it to Microsoft unless you have the Enterprise edition.
Pirates are going to find more keys that are blacklisted and Windows 7 and 8.X upgrades will be invalidated because they don’t have a key. You need to use your 7 or 8.X key to activate Windows 10 now.