The new user manager 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. From 1.6 forward 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.
Updating your site and extensions when needed is easier than ever thanks to installer improvements.