Haven't the foggiest, haven't done anything with it yet. When they release the first beta next month I'll half-inch a spare laptop from work and try and use it in earnest (with Office 14 and VS2010), but for now I've just been exploring.
The new Taskbar's not all there yet in this build, but it's an improvement already. You can move the buttons around, which I've wanted since Windows 95, so that's good. You can pin a running application to the taskbar, and it creates a shortcut with whatever start-up parameters were specified. Not a lot else, and the Start Menu appears to have only changed cosmetically.
Windows PowerShell v2 has a new graphical IDE-type thing, which is very cool, a lot like those Python or Ruby tools you can get (immediate execution window, all that stuff).