If you have used other CMS before, you may find that the Drupal core features have some aspects in common with other CMS. Some examples are the user account registration, user account maintenance, RSS feed, menu management, a method for customizing page layout, and even system administration. This means you might find it easy to adapt the Drupal for your needs no matter how simple or complex. A simple application of Drupal could be making a blog site with multiple users while a more complex application is developing a community discussion forum. This may be why this CMS is useful for important institutions like the US government.
The Joomla! Extension Directory offers more than 6,000 commercial and free extensions that you can use or you can turn to other sources to get even more extensions if you prefer.