Using CTRL+Tab to jump between open tabs now has graphic overlay, much like the Windows Vista ALT+Tab program switcher. The Geode geolocator plug-in has been rolled into FF3.1. The Geode drop-down worked for me, but it wasn't always able to find my location.
You can now restrict searches in the Awesome Smart Bar using special characters such as "+" and "#". These and other aspects of the character restrictions are customizable through about:config.
Two much-anticipated features did not make it into this beta. Private Browsing, Mozilla's answer to Google Chrome's Incognito, and enhanced session management features were not available in this beta.
I found that several of my plug-ins were not compatible with the new tab switcher, so I just disabled all of them. Using a plug-in to force forward-compatibility in other plug-ins can lead to instability and crashes, arguably even more so because this is a beta. However, if you're interested in getting the fastest version of Firefox that we've seen to date, Firefox 3.1 beta 1 is going to deliver the goods.
You can read the full list of changes here.