It's interesting in that this is almost the inverse of something that we have had on the table before. I need to think on this. Very Google+, isn't it?
Hmm, It's like G+ in that you give sharers in addition to sharees control over who to share to. It's not like G+ in that I wanted to be able to post lots of programming posts (say) without swamping my mom's feed. There's still no way to do that in G+. I actually spent some time exploring ways to build this feature. The way I envisioned it, when you went to a user profile you'd see the hashtags they are advertising, and you'd be able to subscribe to some or all of them. But hubski's ability to ignore tags is perfect for this. It seems to me that minimum_wage's comment here addresses your use case, syncretic. What do you think?