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.
- An Admin role, based on Drupal user roles, that automatically receives permissions for any roles added subsequently
- A permissions API
- Fields (think CCK), including file, taxonomy term and image fields
- Rendered output is now returned to the callback function as an array instead of a string
- Securely install and update modules from within Drupal
- Poor Man's Cron
- Modules can alter URL's
- 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.