First Floor Software have published a complete diff of all the assemblies and types between the Framework that comes with the old Windows Phone SDK and the new Windows Phone SDK.
The largest chunk of assemblies removed from 8.0 seems to be in the absence of the XNA framework, and certain Xbox-Live gamer services. There are many many assemblies added to 8.0, essentially making it a much more complete .NET framework. This is probably a by-product of the NT kernel in Windows Phone 8, which allowed them to port over a larger chunk of the desktop .NET. Hopefully this means more compatibility with many .NET code snippets and controls out there!