Allows for maximum flexibility and granular control. It is a framework, more than just a CMS. It can be used for much more than just maintaining content. All Drupal modules are free and open-source, and the Drupal community puts a great emphasis on not duplicating functionality. This fosters quality, well-tested code; code is tested both by automated tests and by members of the Drupal community.
A great blogging platform that can also handle many CMS functions, although not with the level of robustness or granularity offered by Drupal in terms of fields, views, and interconnectedness/reusability of page data. WordPress plugins are usually "freemium", and most decent WordPress sites will need to purchase at least a couple commercial plugins. WordPress plugins aren't subjected to quite the level of scrutiny that Drupal code is, but there are some really great plugins out there.