This new version has a lot of enhancements including the most wanted navigation menu support, custom headers, custom background, shortlinks support, and better use of custom post types and custom taxonomies. WordPress 3 admin interface is lighter and faster with less colors to let you focus on content more. Updating plugins & even themes got easier; you can update all the outdated plugins with a click of a button!
You can view the release video trailer below for more tips on the new release:
You should also watch Matt's talk at WordCamp San Francisco on the announcement page for a brief history on development.
You can view more information on WordPress 3.0 on the WordPress Codex.
So, What are you waiting for? Go update your blog!