From the release notes:
After one year of development we are ready to release Drupal 6.0 to the world. Thanks to the tireless work of the Drupal community, over 1,600 issues have been resolved during the Drupal 6.0 release cycle. These changes are evident in Drupal 6's major usability improvements, security and maintainability advancements, friendlier installer, and expanded development framework. Further, from bug fix to feature request, these issues follow-through on the Drupal project's continued commitment to deliver flexibility and power to themers and developers.
