Great question. I'll try and have some of the Foundation team hop on this, but a few things stand out to me:
- The CSS is over 50% lighter than v5
- New Flexbox grid
- Motion UI Sass animation library
- The Yeti Launch desktop app makes it super simple to spin up a new project
- Each component has been re-coded for Accessibility standards and includes instructions on how and why to use them to meet accessibility standards.
There are loads of other improvements, but those really stand out in my mind.