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JustGoneNow Well, we have yet to see exactly what Andrew, Nuno, and co. are actually planning. But theoretically once posts are tagged it should be possible to easily specify that you want a list of posts that have tags x, y, and z, but not p, q, and r. So if someone was interested in something widely regarded as a fetish but wants to avoid those discussing 'watersports' could specify something like:
+fetish -watersports
It could even be made interactive with a tag cloud so you could click on one tag and that would reduce the cloud to all the tags that occur together with it. Then you choose another and reduce the size of the list each time.
I think that anyone should be able to tag posts even if they are not their own because a lot of people will not bother. And of course a bit of machine learning could suggest tags once there is a reasonably large body of tags and perhaps assign tags automatically.