In Joomla! 1.5, you'll notice:
Substantially improved usability, manageability, and scalability far beyond the original Mambo foundations
Expanded accessibility to support internationalisation, double-byte characters and right-to-left support for Arabic, Farsi, and Hebrew languages among others
Extended integration of external applications through Web services and remote authentication such as the Lightweight Directory Access Protocol (LDAP)
Enhanced content delivery, template and presentation capabilities to support accessibility standards and content delivery to any destination
A more sustainable and flexible framework for Component and Extension developers
Backward compatibility with previous releases of Components, Templates, Modules, and other Extensions