Here's a radical proposal for 2012. How about any topic with below a certain number of posts per week/month, being moved to to a Mumsnet Dumper and they have to be lobbied out, or let nature take it's course after an announced amount of time, and then they get ousted to an archive. Anything that's just become a one-thread clique-zone can be moved to an active "club" section.
I'm not on here that often and have occasionally posted in what are clearly niche areas, which have all the excitement, urgency and dynamism of a game of chess by post, even though as topics they are worthy and interesting. Could of course just be me and my tumbleweed tendencies. Oh yes, and before anyone says it (who do you think you are?/ you don't like it, eff off/ don't give a sh*t mate) it's just an idea and a lot of people seem to complain about the maze of topics.