Joomla only supports one Section and one Category for each content, while you can assign Drupal contents to several Sections/Categories.
Joomla does not support multi-site setups, whereas drupal supports multisite setups.
Drupal has built-in forum discussion, so you don't need to install additional modules.
The term Blog
in Joomla is not same as blog in Internet dictionary. 'Blog' term in Joomla is actually a teaser view of contents containing: Title, Introduction and a Read More link. So, in short, 'Blog' in Joomla terminology is not 'Weblog'! If one is asking if Joomla supports a 'Blog' by default, then the answer is yes, but with a different meaning.
Comments on contents are not available in Joomla by default, but Drupal supports comments for all content-types by default.
