It appears that the header and toolbars have been shortened yet again to provide even more real estate for the web page. The curved corner of the tabs now removed to give it modern look I think. You also see few more links at the bottom of the page of new tab.
Microsoft also said that they wanted to let the UI get out of the way and let the page content be front & center. In comparison many of the UI improvements are small and almost not important.
As Ghacks writes of the actual guts and performance of the browser, as follows:
Improvements have been made in the HTML5 test where the browser now scores 116 points which is an increase of 20 points from the public beta build. The result of the Acid3 test on the other hand has not changed, it is still 95 of 100.
Hopefully, Microsoft keeps making the browser lighter and faster and stays with the minimalist user interface theme. I’ve already found myself defaulting to IE9 beta when I’m browsing and if they can improve on reliability, I’ll make the switch exclusive.