This major release is a long time coming (~3 years).

While the list of new features is long (see CHANGELOG.txt) in my opinion the biggest improvement is the commercially sponsored user experience additions.

Out of the box it still feels a little rough to me but there are some interesting Drupal distributions in the Drupal 6 space (Open Atrium, Open Publish and more) that I expect to really shine as they and others move to Drupal 7.

(disclosure: I build Drupal sites)