For me its hard to think of any other than the obvious customization which can meet special/specific needs which otherwise is not generally available with off-shelf approach...
That said with Wordpress and Drupal you also have tons of options for customization such as developing your own themes, plugins etc... As its open source you get the advantage of a large supporting community, tons of free stuff such as plugins, scripts, themes as well as paid professionally developed stuff - all of which you can leverage and enhance... Further these CMS are maintained and improved regularly such as security updates etc...
Tit for tat there is no doubt in my mind open source CMS such as Wordpress, Drupal, Joomla win hands down...