Very often it's obvious a thread is going in a
direction that isn't interesting to the reader (e.g.
away from the story's main topic, pointless flamewar).
It would be very convenient if one could, say, scroll
back up to the top of the thread (often a +5, first
post) and click icons (e.g. [-] and [+]) that can roll
up/down the whole thread.
What might be even more interesting and useful (and may
even save some bandwidth) would be if users could
choose to read articles with all parent posts rolled
up, to begin with. Then one could open up an
interestingly titled thread -- say, with child threads
still rolled up -- at will.
Conceivably, a reader could digest multiple-pages-worth
of comments in much less time.