IE9 bets heavily on HTML 5
Microsoft has made it clear that they're focusing heavily on HTML5 with IE9, with related features ranging from the genuinely useful to the 'gee whiz that's cool.' If you play around with the DOM Range & Selection demo, for example, you can experiment with changing the properties of a word, sentence, or entire paragraph in real-time, such as changing the text color, size, or font. For the most part, that falls under the 'gee whiz' category, but also underscores how HTML5 can be used to help websites act more like applications than static pages in cyberspace.
Microsoft is siding with H.264 instead of OGG
Because no one can agree on what codec HTML5 should incorporate, it's going to be up to each browser maker (and website) to decide what to use, and Microsoft has chosen h.264. IE9 will support h.264 video, which will probably emerge as the standard most major sites gravitate towards, even though there are compelling cases for going with the open OGG Theora format. On the audio front, IE9 will also incorporate audio tags for listening to embedded MP3 and AAC audio.
Standards compliance will be improved It's a little ironic that Microsoft has gone on record as saying "web browsers should render the same markup -- the same HMTL, same CSS, and same script -- the same way." It wasn't until IE8 that Microsoft appeared to put any real effort into standards support, and as a result, they broke the web in the process. Because so many web developers coded their pages to look correctly in IE at the expense of clean code, Microsoft had to include a "Compatibility Mode" in IE8 so those same pages wouldn't appear busted. In a way, Microsoft made its bed, and now has to sleep in it, just like every other web browser has been doing.
For what it's worth, Microsoft is once again touting a higher commitment to web standards compliance, and has vow to make IE9 compatible with CSS3, which is currently under development. CSS, or Cascading Style Sheets, defines the look and formatting of an online document, and the main benefit here is that webpages will look the same across multiple browsers.
Catching up in performance