The best things about Drupal are its extensibility and plug-ability. You can get something going quite fast that’s feasible for clients to see.
It has a very good authentication/authorization system. For a few years, the application had a dedicated security team who took care of reported security issues with the Drupal core and even contributed modules. The vast user base is a big bonus. You have plenty people collaborating on many modules/plugins, so if you have issues, someone will step in quite fast to help you.
A lot of functionality has been added to the core since Drupal Version 6 allows for people who don’t have a strong technical background to build complex displays of data and pages just by using a WYSIWYG editor.