I've lately been sending people to Wikipedia for information on what's new in Windows Vista, but with the release of the Product Guide on microsoft.com I'll be sending them to this download location now.
A comprehensive feature-by-feature guide to Windows Vista. With contributions from people who actually worked on the product, this is the definitive resource to learn about Windows Vista. The Windows Vista Product Guide is available in the new Microsoft Word 2007 or the new XPS format.
Download it from here.