With the recent release of the IE 9 Platform Preview 6, Microsoft is hoping to keep another just as (or maybe even more) important audience interested, namely developers.
And it's very clear that this release is focused solely on developers. In fact, this one, like the previous developer preview, doesn't even have the full browser interface, just a simple toolbar.
Like all other modern beta browsers, Microsoft's IE 9 is also continuing to push towards support of the not-yet-a-standard HTML 5. Among the new HTML 5 capabilities in this preview is support for semantic elements, which makes it easier to define, link and display semantic content within a website.
The IE 9 Platform Preview 6 also includes support for CSS 2D Transforms, which allow for improved display and handling of elements, something that developers can use to build richer and more interactive web application interfaces.
If you're not a developer there really isn't much reason to use this preview and the lack of a full interface makes it not very friendly for regular users (though there is a somewhat complex and time consuming way to enable the IE 9 beta interface for this preview).
If you are a developer, or just a really curious user, you can download the IE 9 Platform Preview 6 at http://ie.microsoft.com/testdrive/.