you want an interesting ratio? try my ratio of threads started to threads deleted!
Seriously though, the only outcome of limiting the post count in a thread on a topic will be a plethora of other threads on the same topic. Remember that when a thread stops getting posts it will drop off the front page and when that happens, the new thread starter (who NEVER checks for the existence of other topics before posting) will just start a new one.
Net result is that the same 20 topics will keep getting recreated and re-argued by the same people smacking down the new members that wanted to go around again.
The advantage of the mega thread is that if the topic has any value it will still be on the first page or so and therefor limit the number of recreates.
(I agree that we need to do more about some of the multi-thread nutcases here - we ARE trying to take care of them. Just have a look at the efforts that Francois has made in the clinic!)
