This latest version not only addresses over 790 bugs, but also changes the way it does styling and scripting making it much easier to use. In addition, you can now add a theme with one simple click.
The widget interface has been completely redesigned make that simpler and developers have a 'more robost API' to work with.
Overall, there are approx 180 changes in the codex that can be reviewed here.
WordPress 2.8 is available now and can be downloaded from Wordpress.org here.
Important Update 1: A few early users have been reporting some 'difficulties' on both Twitter and Friendfeed. A good piece by Duncan Riley can be found here.
Updated 10:30 PM ET 6/11/2009