There are some new features or improvements:
* WordPress Widgets allow you to easily rearrange and customize areas of your weblog (usually sidebars) with drag-and-drop simplicity. This functionality was originally available as a plugin Widgets are now included by default in the core code, significantly cleaned up, and enabled for the default themes.
* Full Atom support, including updating our Atom feeds to use the 1.0 standard spec and including an implementation of the Atom Publishing API to complement our XML-RPC interface.
* A new Blogger importer that is able to handle the latest version of Google’s Blogger product and seamlessly import posts and comments without any user interaction beyond entering your login.
* Infinite comment stream, meaning that on your Edit Comments page when you delete or spam a comment using the AJAX links under each comment it will bring in another comment in the background so you always have 20 items on the page. (I know it sounds geeky, but try it!)
* We now protect you from activating a plugin or editing a file that will break your blog.
* Core plugin and filter speed optimizations should make everything feel a bit more snappy and lighter on your server.
* We’ve added a hook for WYSIWYG support in a future version of Safari.
Here you will find the full announcement.