Here's a list of highlighted changes:
- Performance (using something called Direct2D system to improve client-side rendering
- Richer web support (more rounded corners available via css3)
- Support for HTML5
- Faster Java Script engine
While Microsoft boasts IE9 will show performance improvement, it still lags behind the competing browsers. Here's a graphic depicting a benchmark of performance:

It amazes me that Microsoft has been in the browser business for so long and can't compete against a new comer like Google's chrome. It goes to show it's not their niche. I give them credit for trying to stay in the game though and it's high time they improved on IE8 which is almost unusable compared to the other browsers (in my opinion). The graphic above says it all. Given the release of this browser version is at least a year out, Microsoft may throw more bells and whistles in there. They will need to release this version somewhat soon if they want to remain competitive. There are many users of the current IE version who already have a sour taste in their mouth. For more information from a developers view, you can visit http://blogs.msdn.com/ie/archive/2009/11/18/an-early-look-at-ie9-for-developers.aspx

