Dropdown menu with descriptions only in sub-menu elements in Drupal 7?
Within your theme's template.php, you can override the theme_menu_link() function. 

Target the $variables['element']['#original_link']['depth'] variable to check for the depth of the current menu item. 

The variable $variables['element']['#original_link']['localized_options']['attributes']['title'] holds the "description" value entered on the menu item's edit page.