Major new features in this release include a sexy new default theme called Twenty Ten. Theme developers have new APIs that allow them to easily implement custom backgrounds, headers, shortlinks, menus (no more file editing), post types, and taxonomies. (Twenty Ten theme shows all of that off.)
Developers and network admins will appreciate the long-awaited merge of MU and WordPress, creating the new multi-site functionality which makes it possible to run one blog or ten million from the same installation.
As a user, you will love the new lighter interface, the contextual help on every screen, the 1,217 bug fixes and feature enhancements, bulk updates so you can upgrade 15 plugins at once with a single click