So in other words, Circles are backwards from Twitter, and this is a problem.
I guess G+ needs a mechanism to allow people to ask to subscribe to a Circle, and the owner can approve or deny it. (Or set an option to auto-allow.)
That keeps the privacy aspect, but also makes it easy for people to follow or be followed.
If that happened, then the circles themselves would become the tags.
Another post here on HN talks about Circles not being Groups. (The Zuckerberg one.)
Reposting my comments from there:
I would definitely like to see Google add groups as pseudo-people... And then you can put that group in a Circle and share to it.
People in the group can see who else is in the group. Groups could be curated (need approval to join) or just open to anyone, and moderated (need approval to post) or not depending on the choices made by the group owner.