As with earlier previews, one of the major features to compare is hardware accelerated HTML5 animations and audio. To help you test, the IE platform preview page has a number of demos that you can run side-by-side in various browsers to view performance and rendering characteristics for each.
Oh, and one other big piece of news for those who are fans of the Acid3 test. The Internet Explorer 9 fourth Platform Preview scores 95/100 on Acid3. You can find more discussion of the new features in the fourth Platform Preview, as well as some notes around Acid3, on the IE team blog here.
TIP: for folks who use the page zoom feature in Internet Explorer 8, the IE 9 platform preview will pick up that setting, so if you have IE 8 set to larger than 100% zoom, and you want to view pages at normal zoom in IE 9, just change the zoom level in IE 8 prior to opening the IE 9 platform preview. Presumably, once a version of IE 9 with the full browser chrome has been released, it will have its own separate zoom settings.