Fastest Drupal ever, out-of-the-box. Comes with BigPipe, Placeholdering and other crazy stuff, and a better caching system.
New theme system based on Twig. 154 theme functions have been removed and replaced with Twig templates. This is one of the most interesting additions to the theme layer.
Symfony2 as a base layer for Drupal 8. Services, Dependency Injection and Routing are some of the paradigm shifts.
Everything is fieldable, well almost everything. Not only are there new fields, but you can now add fields in many more places.