I have used a few different content management systems (CMS) from Mambo Server (horribly painful) to Interwoven TeamSite but none of them come close to Drupal.

1. Drupal makes sense to non-technical users.
2. Drupal doesn't get in the way of technical users.
3. The community is polite, helpful and well organised.
4. The more I learn the more I appreciate how well thought out Drupal is.
5. Drupal is a tool I look forward to using.
6. Drupal gets the job done with minimal fuss.
7. Documentation is extensive and of high quality.
8. Using Drupal increases speed of web development more than any other tool I've used.
9. Drupal's module development framework is extremely well executed.
10. There is an extensive collection of modules that are of high quality and have minimal overlap.

drupal.org