I feel that the Human Powered Vehicles section of the site has done exactly that - the contributors, discussion and comments tend to be the same "converts" and whilst their contribution is valuable there is a lack of fresh and questioning input.
when it was set up it wasn't for recumbents only - it was for all of the weird and wonderful none-standard types of cycle (folders, trikes, tandems, small wheelers, cargo bikes, etc) but seems to have become the sole domain of 'bents, thus it is visited and contributed to by only a few. Perhaps a different name might have helped - e.g. "Non-standard Cycles" or some such.
And you could say the same about the MTB section.
But I guess that it's a bit of a fine line: either you have minority interest topics in with everything else and those interested in reading them find them hard to locate or keep near the top, or you segregate them off and only a few people visit plus they don't attract newcomers. Although, of course, getting someone interested in a new type of bike is a little different to getting them interested in a new sexuality