I upgraded from Windows 8.1 Pro N to Windows 10 Pro N two days ago. The only issue was that I needed to reinstall my Nvidia graphics drivers.

FFXIV would not start initially, but installing the Media Feature Pack for N and KN versions of Windows 10 fixed it. Seeing as N and KN versions have been around since XP days, I'd call that shoddy programming on SE's part.

DX11 seems to work fine with no noticeable changes.

Windows 10 seems better than stock Win8.1, but possibly worse than Win8.1 with the third-party Start8 start menu - I needed to manually add my system-wide start menu to the indexing service to get search to work for those programs.

I am prepared for a clean reinstall, but I don't think it'll be necessary. It seems to start up slower, but otherwise seems just as fast as Win8.1.