The new user manager which will let you manage who has access to what in your site. You can leave access groups exactly the way you had them in Joomla 1.5 or make them as complicated as you want. You can learn more about how access control works in on the Joomla! Documentation site
In Joomla 1.5 and 1.0 content was organized into sections and categories. In 1.6 sections are gone, and you can create categories within categories, going as deep as you want. The sample data provides many examples of the use of nested categories.
All layouts have been redesigned to improve accessibility and flexibility. If you would like to keep the 1.5 layouts, you can find them in the html folder of the MilkyWay template. Simply copy the layouts you want to the html folder of your template.
Updating your site and extensions when needed is easier than ever thanks to installer improvements.