There have been so many changes not just under the hood but to the admin Interface as well that it is impossible to list them all on this page.
Highlights of some of the important changes are:
- Improved UX
- Views in core
- Content translation in core
- Web text editor (based on CKeditor)
- Inline Editing
- Configuration management system
- Symfony PHP framework
- Object Oriented PHP
- RESTful web services
- Composer package manager
- Twig template engine and YAML
Visit: https://www.drupal.org/8 to find out what improvements have been made for site builders and developers.