Object Oriented Programming
Drupal 8 now adopts modern PHP concepts and standards, object-oriented programming (OOP), and the Symfony framework. The shift from procedural to OOP is a big one, especially for those that develop modules in Drupal.
More functionality out-of-the-box
Popular modules in Drupal 7 have been ported into Drupal 8. Some modules include:
Other features in core, such as Ckeditor, URL and Date fields, HTML5 mean those related contrib modules are no longer required. Drupal 8 core is also much better out of the box for foreign language and multilingual sites.
Drupal 8 is a mobile-first platform with a whole bunch of mobile-friendly features added to it. Out-of-the-box, Drupal 8 works on mobile with the default themes (Bartik, Seven & Stark) now being responsive. Also, in core is a responsive image module that allows a page to only load images that have been specifically sized for the screen based on screen width breakpoints.
Drupal 8 has a new configuration system designed to improve exporting and importing site configurations.