Short version: It rocks!
For users, WP 3.0 offers a new default theme, Twenty Ten, that allows for customizable headers and backgrounds and support for drop-down menus out of the box.
For developers, WP 3.0 provides three ginormous new features that make WP 3.0 significantly more powerful than earlier versions.
- custom page and post types
- custom taxonomies
- multiuser support
WP was already a pretty robust content management system, but one of the things it lacked was support for custom content types. Users were limited to only pages or posts. While you could often fake it using excerpts and/or custom fields, solutions were often unwieldy, and managing them were troublesome for both users and developers.
WP 3.0 changes that. Now, in addition to posts and pages, you can create you own content types — movies, for example — with their own associated fields, such as actors and ratings.
Six Revisions has a run-down of what’s new. Mashable has the highlights.