About a year ago, we talked about something similar to this: unique tags that could be shared with specific users. Basically user-specific tags but with multiple users. Jesus, there have been a ton of weird variations that we have discussed. One that creeps back into my head from time to time is 'follower only posts' and/or 'follower only' messages. Only your followers can see them. I don't think these are good ideas. The possibilities from variations on what we have are dangerously large. But, I suspect some gems are in there.
HULK SMASH UNIQUE TAGS If I want to publish something that belongs only to me, I'll write a blog. This whole mental exercise erupted from the forehead of Zeus because thenewgreen asked me if I wanted a poison pill for my followers function. I said I'd rather have presets. Then I decided that presets would be even cooler if they were communal. It's pretty much the exact opposite direction from the way you guys are going with user-specific tags.
I like user specific tags because I can stop following "music" which often contains a sleuth of content I won't be interested in and follow #music.flagamuffin or something like that.