The Beta release addresses lots of issue which the IE8 was still struggling from, like HTML5, Tear-Off Tabs & Acid Compliance(Chrome has 100 percent but IE9 is at 95 percent but its still in the BETA)
Those are just the major changes, they have a long list of Tests from categories like Performance Demos, Graphic Demos, HTML5 Demos & Browser Demos you can view all this and other Demos on Microsoft Internet Explorer 9 BETA Test Drive Website.
IE9 BETA also has hardware acceleration to further enhance the browsing experience you can all that on Beauty of the Web
Even better, sites can choose to code jump list items for those pinned sites, as you can see in this shot from the Amazon jump list:
Another good addition is that sites can choose to code Jump list items for those pinned sites, as you can see from the image of Amazon Jump list: