For the best improvement is CCK, support of images, localization, in "stock" version + improved and better UI with new admin skin. Only WYSIWYG editor is missing.
See this for a complete list of features http://drupalcode.org/viewvc/drupal/drupal/CHANGELOG.txt?revision=1.296&view=markup
Drupal 7 is going to be sweet, sadly based on the current backlog of crtical issues theres still another five months before we'll see a release.
I know it will be awesome, but I'm satisfied that it is isn't released too early. I have lots of websites under Drupal 6 and I'm not looking forward to upgrade them to 7. As for new sites I'll start in the meantime, waiting for Drupal is actually a good excuse for myself
Better upgrade to Drupal 6 because drupal.org has released this announcement. Drupal 5 will no longer be maintained when Drupal 7 is released. Upgrading to Drupal 6 is recommended. Code (markup):