Did I remember to mention expandable/collapsible comments (affecting their appended threads, of course). Giving commenters the ability to post default-closed comments (default-open to their recipients) would add a useful dimension of expressiveness, enabling one to say something that is a distraction from the main thread, but appropriate in a more local sense.

A further refinement would be to provide a rating dimension that can change default-open to default-closed for subsequent readers. This would be far short of calling "Troll!", but would gently say, "This doesn't really fit here", and move the comment out of the way.

Words and ideas I offer here may be used freely and without attribution.