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.
Drupal is a free open source framework and Content Management System (CMS) created with the programming language PHP. It has been utilized in building many different types of websites, ranging from small personal blogs to large corporate and political web portals.
Somewhere Dries said that Drupal powers more than 1% of the Web. Most of these sites are probably small business Web sites with moderately basic needs. One of the great things about Drupal is that you can be a blog today, and with a few new modules, you can be a full eCommerce solution tomorrow.
Popular brands use Drupal to build their websites: