I upgraded to WordPress 3.1 this morning and besides the large list of security/bug fixes (over 820) there are some neat features that are worth noting. The new admin bar is really slick and allows a lot of functionality that would have previously required you to go through the admin panel. Via the admin bar you can add new content, adjust widgets/menus, provide information about updates, work with SEO information and etc. Just the admin bar by itself makes this release a worth-while upgrade.
Several improvements to how links are handled makes for a greatly improved work-flow when linking back to old content which at the end of the day saves time creating new content and that is a very good thing.
I also notice a very nice boost in performance w/ WordPress 3.1, both on the admin side as well as the client side. I imagine for that really large sites the difference is even more noticeable. WordPress is feeling more and more like a fully-featured CMS with each release and I am very excited to see where 3.2 takes us.