My collegue David Chou does a great job of distilling the new platform, here is an excerpt... 

Collectively, the rich cluster of "Web 2.0" sites on the internet form a services foundation from which applications and functionalities can be built upon, without needing any additional dedicated infrastructure. As a result, this collection of cloud-based services form a new kind of "platform" to create a new breed of applications. David Chou provides a perspective on what “Web as a Platform” means in the Web 2.0 world, and a generalized view of the cloud-based platform. Check out the full blog entry here.http://blogs.msdn.com/dachou/archive/2007/11/25/web-2-0-a-platform-perspective.aspx