I've tried the Windows Developer Preview:http://www.youtube.com/watch?v=FRixbw_0Etghttp://www.youtube.com/watch?v=uf0xGhNkePshttp://www.youtube.com/watch?v=ge3Ctn5K7gA
Some of the major changes I know of:
-Full integration with Windows Live (Hotmail, MSN, etc.) because once you type in your hotmail address and your password you get your picture, and some settings, customized for you...
-Metro UI: the Metro userface is the complete rework of the Start menu and the login page, among other things.
-Built-in Windows Defender provides full protection against all viruses, not just spyware like on Vista and 7