Boiling down the zillion hours of D7 development into a short list of important changes isn't practical, so consider this a starting point rather than a comprehensive list.

1. AHAH functionality without touching the underlying JavaScript
2. An Admin role, based on Drupal user roles, that automatically receives permissions for any roles added subsequently
3. A permissions API
4. Fields (think CCK), including file, taxonomy term and image fields
5. Rendered output is now returned to the callback function as an array instead of a string
6. Calls to allow the orderly editing of added page Javascript/JQuery
7. Securely install and update modules from within Drupal
8. Poor Man's Cron
9. Modules can alter URL's
10. File and Storage API for robust file handling

And a bonus to make it a baker's dozen. An mention of changes that spell huge additional promise for Drupal ... support for RDF and a CDN!

Based on the article More Than 50 Drupal Modules Moved Into Drupal 7 by Daniel F. Kudwien.