Massive modernization. Most high-level concepts remain the same. The implementation became much better. And it's a joy instead of a pain to extend/customize.
I recommend you to take a look :) See https://www.drupal.org/drupal-8.0 — RC1 was tagged _yesterday_.
It sounds like you will particularly like the Configuration System. See https://www.chapterthree.com/blog/principles-configuration-m...