I just pulled the Framework Design Guidelines down from the bookshelf to look at it again. I briefly browsed through it and I must say that disagree with it all the same places as last time I read it. At one point they recommend the use of abstract classes over use of interfaces when making a framework. The reason would be that it's easier to add new methods to the framework later without breaking code build on the framework.
I recommend interfaces over classes when designing a framework. Interfaces give fewer dependencies if you have the choice. Like everything else its dependent on the situation what to do.
The book states what you should do and what you should not. Framework design to me is not so black and white. The book is probably more a description of what practices the .net team followed while designing the .net framework. The book is good for developers with little experience, but it seems a little trivial for experienced developers.