I think that it’s important to be clear on ownership.
Only the author of some content should have the ability to tag his/hers content. Using that logic, we could maybe treat comments as posts - with their own set of searchable #tags but still linked to their post (kind of like twitter).
While searching, one could then search for recent posts and/or comments with tags in OP and/or comments.
(From Paul) This mainly solves the problem of abuse as it makes it very clear who is responible for what content.
The problem with abuse is real and as said, a problem… Deleting the comment is a quick solution, but what if the post owner isn’t online for a few days and comments with tags get added that he doesn’t approve of?
A solution for this could be, that everybody can add tags to their comments, but the post owner must approve the comments individually first, to make the tags “live”. This way, the post owner is in control of where his post shows up.
What do you think?