It often seems strange to me that of 100 people who read a thread, only 1 actually posts. Or, for that matter, votes. I suppose the problem is that you have to register, but it really is not that hard to register, and it is the only way I've found to prevent spam. I still get more than a dozen requests from spammers every day for membership, but no follow up e-mail that would allow me to activate their accounts.
My own feeling is that forums that have a lot of posters get a lot of posters, which is what I mean by critical mass. If more people would post, then more people would post.