Yep, I want this too, it's mostly bottlenecked on our upcoming editor rework, and after that will be primary determined by how hard it will be to implement this in the new editor, so I sadly can't really give an estimate of when we will have this feature (since I don't fully know the constraints of our new editor framework yet).
As LW is an intellectual **community** where people get to know each other and know "who knows what, who's working on what" it seems like something like tagging people FB-style in specific posts/comments is a very low-hanging fruit. (Of course you should be able to silence people from tagging you or sort taggers by some score). Is this in the pipeline, or has been ruled out for some reason?