I want to list all the pages in Category:Foo but only pages that are both in Category:Foo and Category:Bar. And vice-versa too, that is list all pages in Category:Foo that aren't in Category:Bar.
Topic on Extension talk:CategoryTree
This page... Extension:DynamicPageList list 2 extensions that do the sorts of things you are looking for, one extension is more basic and simple than the other. Extension:Semantic_MediaWiki adds more features again, but adds even more complexity.
Thanks! I'll give this a try, looks very promising!