Perfect, Kovid. I'd seen the grand tour, but somehow missed the UI tips video. Following your instructions, I've made a new Subjects column, which also appears in the tag browser on the left. This leaves the Series field open for the data it's supposed to contain.
And I aggree 100%, worldwaker, the filesystem is a poor poor substitute for tags. And I don't plan on messing around 'under the hood' of Calibre's file organization in the finder. If I had to choose between tags and my subject field, tags would of course be way better. But as I said, I think I want to use tags for very specific info. The subject field is for general categories of reading materials: literature, history, education, etc. I actually like the fact that I can restrict it to just one entry. It just gives me one more option when browsing my collection.
Extremely impressed with the level of customizability in Calibre. Bravo, Kovid!