I just noted how powerful this feature can be!
I see the situation of a WordPress-based WebSite, with some categories and tags, or a Drupal-based WebSite with all its taxonomies.
If, from its main feed, you can grab each post’s taxonomies (I’m almost sure RSS doesn’t support it, and even if it does, I’m mostly sure most WebSites don’t provide this kind of metadata in their feeds), we could point those categories we like most, and blacklist those we don’t wanna follow! Instead of adding a feed for each category!
Of course, inspecting each WebSite and its posts’ tagsoup for taxonomy is damn hard and resource-consuming. That’s where premium subscription enters!