What sub-category of physics incorporates most if not all other sub-categories of physics?
I don't think there is such a subcategory.
Why does WikiAnswers only have 2 subcategories under 'Philosophy and Philosophers' when there are so many other areas of philosophy?
Well, columnist is just a subcategory of author. A columnist is an author who writes a regular column for a newspaper... like Ann Landers, or that type of thing. Other authors might write novels, or poems, or short stories, or personal essays. They are all authors, but words like "novelist," "poet," or "essayist" would also cover some of the subcategories.