I have wanted that third tier on LJ for a while. I can track the conversations I want to follow, and I can scan back on my friends page to see if there are new comments on posts I was more casually interested in, but the latter becomes a hassle if there are too many posts where I don't care. Filters are per-user, not per-post, so they only help to the extent that interesting threads correspond to interesting posters. If I could mark an entry as "done with this" for purposes of the friends page, while still having it available via the journal (or an "everything" view) if I really wanted it, I would be less likely to miss interesting comments on posts from yesterday (which is forever in LJ time, it seems).