For themers, there are lots of exciting new features in Drupal 8: the Twig tempting system, libraries for loading assets, and the replacement of theme functions with templates. There are also a new set of core themes that come with Drupal 8.
Drupal 8 theming has lots of new features, one of the most exciting is the addition of libraries. You can use libraries to add stylesheets and javascript to your theme, globally or in particular situations.