It's way more standards compliant than any previous version of IE, with a 95 score on the Acid3 web standards test. Like every browser, it's packed in a bunch of HTML5 features, though what Microsoft's pushing the hardest, particularly in its big shebang demos - that you can catch here - is the fact that it's using Direct2D and DirectWrite for hardware-accelerated graphics rendering with your GPU.
The promise is that by tapping your graphics card, the browser can run more hardcore web apps like games in the browser, as some of Microsoft's impressive, if slanted, demos attest to. (Google's pushing its own ways to reach the same end.)
In terms of usage, IE9 is the first time Internet Explorer didn't immediately make me wanna plunge back into Chrome or Firefox - oh the bliss of the unified search/address bar - both in terms of the interface and the way it renders pages. It has its share of wonkiness while it's still in beta. In addition to the weird way it renders our quicklink headlines, for instance, it completely chokes out on Apple's spiffy new HTML5-powered iTunes Showtimes, even though the page works perfectly in Chrome, Firefox 4 beta 4 and Safari.
Google's Chrome Arcade Fire demo totally kills IE9's Killers demo, though. The beta's out for everybody later today. You might as well check it out, since it's what your mum's going to be using in a few months - you might even be impressed. It's right here: [IE9 Beta]