As well as the code changes, such as adopting Symfony components and other third-party code, one of my favourite things about Drupal since D8 is the new release cycle.
Instead of "It'll be ready when it's ready.", since Drupal 8, Drupal releases new versions on fixed dates.
If a feature is ready by that date, it will be included.
If not, it will be included in a later version.
As well as regular bug fixes and security updates, there is a new feature release every six months.
Drupal 8.1 included features like the Layout Builder, which wasn't included in 8.0.
In earlier Drupal versions, it would have been deferred to the next major version, making it longer before users get its benefits.
A fixed release schedule makes it easier for users and Developers to plan their updates and prepare to upgrade to the major version.
I'm an Acquia-certified Drupal Triple Expert with 17 years of experience, an open-source software maintainer and Drupal core contributor, public speaker, live streamer, and host of the Beyond Blocks podcast.