The minor enhancements are the icing on the cake for 2.6. What You See Is What You Get (WYSIWYG) editing is generally improved with better image support, integrated word counts; and revamped TinyMCE. A number of security improvements also have made it in, including improved SSL support as well as some improvements to cookies and database interactions.
Anytime a significant upgrade to any software package comes along, backward compatibility is a concern. With almost 2 million downloads of WordPress 2.5, there's a significant established user base to consider. Fortunately, WordPress 2.6 is fully backward compatible, including plug-ins, so the upgrade path should be straightforward. Fresh installs also are a breeze, as WordPress continues its tradition of a simple five-minute install.
For additional details on the new features, big and small, check out the WordPress 2.6 announcement page on WordPress.org. You also can check out a quick video overview of the changes and enhancements.