Wordpress 3 is a new milestone (literally). The changes are quit drastic and luckily welcome. The following are the notable changes that will surprise most of you:

WordPress Mu (multiuser) has been merged with WordPress: This means multiple sites/blogs on a single wordpress install. We’d been waiting on this for ever. And now we can streamline all our bloated CMS installations into one. This seems to be going the drupal way; impressive!
New WordPress default theme: At last the boring old face of WordPress is in for a welcome change. The default Kubric Theme while being elegant, minimalistic and almost perfect has become a little too boring everytime we installed WordPress on a new site. Thus a new theme. And as the trend goes, this one comes with a nice, wide and beautiful header image. So why a new theme? We guess it’s to handle the new frontend requirements of WordPress Mu… and guess what — BuddyPress we bet.
Menus: Now you can build your own navigation menus in the admin screen.
Background Image: Upload and customize your own background images.
Header Image: Upload and customize your own header images.

While we observed the changes only at a superficial level, there’ll be more that has changed in the backend. WordPress 3 as a result of the merger of the two codebases — WordPress Mu and WordPress — will mean newer bugs and support tickets like any other major software release without exception. The changes may also mean headaches for the theme developers and the older v2.9 compatible themes because they may need to be updated to support the new Mu backend (and maybe buddypress). While WordPress 3 is only at beta 1, a lot of what it will become and its stability will mainly depend on the feedback from early adopters and beta testers. Keep watching this space for a complete review of wordpress 3.0 as events unfold.