I tend not to understand many of the improvements they rave about at WWDC. It's mostly improved versions of stuff I never use. (Mission Control comes to mind, or Notifications, or some obscure function in Safari).
I also disapprove of the tendency to make everything 'flat', so you can't see the difference between ordinary text and a button. This gets worse with every new iteration of MacOS. What's wrong with the shiny pill fake 3D button? Works for me.
Just wondering if I am missing something important not mentioned in the official Apple blurb, some under-the-hood changes like improved multithreading for real-time apps, better APFS implementation, improved SMB networking, that sort of thing.