Working with multiple template styles and overrides for the same views, creating the design you want is easier than it has ever been. Limiting support to PHP 5.2.4 and above makes Joomla lighter and faster than ever. Languages files can now be overridden without having your changes lost during an upgrade. With the proper xml your users update extensions with a single click.
Access control lists are now incorporated using a new system developed for Joomla. The ACL system is designed with developers in mind, so it is easy to incorporate into your extensions. The new nested sets libraries allow you to incorporate infinitely deep categories but also to use nested sets in a variety of other ways.
New events throughout the core make integration of your plugins where you want them a snap.
The separation of the Joomla! Platform project from the Joomla! CMS project makes continuous development of new, powerful APIs and continuous improvement of existing APIs possible while maintaining the stability of the CMS that millions of webmasters and professionals rely upon.