1. Caching - The various caching tools enable Drupal to perform nearly as fast as static HTML
2. Community - Lots of IRC participation, local user groups, positive leadership
3. No forking - The community hasn't yet reached the point where a schism caused forking.
4. Central module repository - This has meant that all projects are supported in similar ways (version control, issue queues, etc) and that they are GPL compliant. Also, this adds exposure for the modules and they get vetted by the community.
5. Drupal is a programming framework plus a CMS - Drupal does a lot of heavy lifting, and helps you organize your code in a meaningful way other folks can plug into.
6. Drupal modules don't have to hack core to work - As opposed to other CMSs
7. Drupal scales well - And this will only get better with Drupal 7 and the new database layer
8. Drupal is mature - It's been around a while, it's stable and is being supported by a lot of big projects (need some good diagrams here)
9. Extendable data structures (i.e. fields in core) - Makes all data flexible at the interface level
10. Views, is one very strong point for using Drupal, no other CMS has anything remotely comparable to views functionality.