Even though we are entering the gray area (where a forum starts discussing the inner works of the forum more than the interesting outside matters), i think this is a good point.
I've noticed this problem myself with the
giant GridMove thread, that became so big that any information in it is pretty much useless now.
I think app was "right on the money" there:
When you compare the discussion on page 15 to page 1 and find they are totally unrelated.
-app103
4 out of the 10 longest threads on DC are development-related (5 if you count the "General brainstorming for Note-taking software"). This makes me think these are the threads that tend to grow and grow and grow.
I think it's a shame that happens, because even though it eases the development, people just end up repeating stuff that was already said in previous posts (i recall a specific post about gridmove that i had to link to 5 times, because that question constantly popped up).
Still, it's interesting how all the threads have managed to respect app's "not too long" rule, after all those posts they keep on topic (at least most of the posts do).