Carleas, might I suggest that on the main page that lists all of the different forums, that you only display the titles of the first few forum posts made by the members, and leave off the pinned posts.
You can still have the pinned posts, but they will (as they do now) appear at the top of the designated forum after clicking on that forum.
I personally think that certain forums (such as Religion and Spirituality, for example) would attract more attention if the members (and the visitors) see the title of a new (hopefully - interesting) thread topic, as opposed to just (as in only) a stale list of pinned topics.
At the very least you can make sure that at least one or two of the newest - member-generated thread topics - appear below the pinned topics of ALL of the forums on the main page.
