Ars Technica walks us through the various improvements in the user interface. Please note that these changes are not yet part of the build handed out to attendees of PDC - these changes are in the newer build shown during the keynote.
The most prominent change is the completely new taskbar. Entries are now strictly per-application, and hovering over them will show thumbnails of the application's child windows. Taskbar buttons lack text - they are icon only - and you can (finally!) rearrange and pin them in whatever way you like. Another new feature are jump lists:
Microsoft also made a number of changes to window management. Dragging a window to the top of the screen will maximise it - dragging the titlebar down will restore it. Dragging window to the side of the screen will make them take up 50% of the screen, so you can quickly get two windows side-by-side. These changes are there to reduce the number of clicks, and are based on Microsoft's data.
You can also 'peek' at windows, so you can see their contents without actually switching to them. By scrubbing your mouse over a obscured window's taskbar entry, al other windows will turn into glass outlines, allowing you to see what's going on in said window. This also works for the desktop, and is handy since gadgets now live on the desktop (the sidebar is gone). A sort of Aero glass variant of Expose.
Microsoft listened to its users when it comes to one of the most hated parts of Windows: the system tray. Effectively, it not longer exists. Scrap that - it's still there, but each new entry into the system tray is hidden by default, and must be specifically enabled by the user. Personally, I use WinPatrol which more or less achieves the same. Microsoft also made the regular system tray entries (wireless, sound, etc.) more useful by allowing you to do more with their menus (like switching wireless networks).
Another change concerns Windows Explorer, the file manager. While WinFS may never come to fruition, its features do find its way into Windows 7: you can create arbitrary libraries of files (which look like toppled-over stacks), with default sorting criteria already available (for instance, libraries by year).
The idea I'm getting from all this is that Microsoft is working to really refine the Windows interface - which was sorely needed as it had become quite apparent that what worked 13 years ago, didn't work any more today. While it's hard to tell without actually using it, it seems as if the company is working towards a more unified and less cluttered interface that requires fewer clicks to do stuff.