Thanks, @tomalmy: I knew about AND / OR / NOT operators in Spotlight, but I failed to mention it, so thanks for that follow-up note.
What is cool about Leap.app, as you may know, is that you can add tags to the search by clicking on them in the tag cloud, and the tag cloud shows only the tags in the current search scope, so since the search scope changes each time you add a tag, the tag cloud changes to show only the remaining available tags. You can navigate to a particular folder, easily exclude certain subfolders from the search scope, and see a tag cloud of only the tags in that particular folder but not the excluded subfolders. That kind of sophisticated tag-viewing/searching logic would be great to have in the Finder, as @JoelAnderson suggested. Fortunately we already have Leap.app.