Here's the nature of the beast, as I understand it.
You have active participation, and passive participation. Active participation is when the member goes out of his normal routine to contribute to the site. For example, signing up, starting threads, starting a project in the community, volunteering to be a moderator, etc. Passive participation would be the routine of responding/replying only - a periodic sweep of the site's recent activity / email subscriptions, and replying to what they see is interesting. Passive participation only generates activity when provoked.
Most of us are mostly passive - and that's great! Better than none. We don't really think about things until we read about it or are somehow inspired to write a post, or sometimes even a new topic (rarely do I think people login to the site with the first objective of creating a thread - most create a thread after being inspired by other posts in some way).
When people think about it, they'd like to see more meaningful posts. But the problem is, they don't think about it - if they did they probably would create a thread. But it's routine to read posts just like you check your emails.
Also, many older members have exhausted things to talk about, or have noticed a few topics that repeat the same subject. New people may have never discussed it, but old people may be sick of it. Besides, when does a discussion actually go anywhere or change anyone's perspective? Do we really learn anything from them? If we have expectations of our contribution (remember that Online Participation thread I talked about with the wikipedia article), and they are not met, then we're less likely to continue participating. Fluff threads we have such low expectations they are easier to meet - but more meaningful threads we generally have high expectations and low yeild.
How do we fix this? Well, it's a rather complicated problem. It'd be nice to have a wiki that conversations can contribute to - that way whenever the subject comes around again new people can read up on it and old people can review, and and constructive conversation picks up where it left off instead of building the 1st floor of a tower over and over.
How do we get more active participation? We don't, really. We make it as easy as possible to passively participate, and try to offer more inspiring content, and a more rewarding experience (such as using the method above).