Yeah, the toughest part would be in implementation and making it easy for people to follow the combinations. There would just be too many permutations of user/tags/domain to follow for every post (to have buttons in the upper right of story pages like there are now). The URL structure on the other hand, seems like it'd be pretty easy, just continue adding on the hub?id=doesntgolf &tag=music &domain=npr.org or whatever. Of course, I have no idea what sort of work it would tag on your end to implement that sort of URL structure.
I really want to think on the concept some. Sometimes these things illuminate unexpected possibilities. It is interesting how just flipping the specification from poster to follower has a psychological effect. It would be curious to see 'personal tags' of mine that I never created that are being followed. As it stands, we would have to wind things back before we could implement this in a way that didn't confuse the heck out of people.