AnandTech has published an extensive Windows Phone 7 Series guide, you can check it out over here.
Let's start at the beginning - WP7S does away completely with everything Windows Mobile. That means Windows Mobile applications won't run on WP7S, hardware running Windows Mobile won't run WP7S (including HTC's HD2), and Windows Mobile is no longer being actively developed. Existing hardware will get support for corporate clients, and the developer tools will remain, but they won't be actively developed. Consider Windows Mobile officially banished from the Microsoft kingdom, and you get the perspective. To give you an example of just how banished Windows Mobile is, there was virtually no discussion of porting applications from Windows Mobile to WP7S - this is a completely different platform. Microsoft wants developers to forget about Windows Mobile and immediately start thinking WP7S. The sense of urgency is because Windows Phone 7 Series will ship before the end of the year ("Holiday 2010").