Nothing major, just did a quick backup then hit the upgrade button.  While things may have changed significantly in the backend code, there really isn’t much to show you here.

• WordPress MU (the multiblog variant of WordPress that is used on WordPress.com) is now built into the maintstream code.  Thus endeth the tyranny of MU, a code branch I personally despised.
• Custom menus make it really easy to create a special nav menu.  I’ve already used this, and it’s a nice feature.
• They finally let you pick your own username and password for the admin user during installation.  Seems like a little thing, but it’s been a system issue since day one with legions of users forgetting to either change the admin user password or to write it down.
• Custom Post Types and Custom Taxonomies – I haven’t used either, but I suspect that I will soon.  Both of these are hardcore CMS functions.
• Scads of new hooks and functions for plugin and theme developers.