Part of the problem as well is that people tend to stick to their fandoms of choice, both as readers and as authors looking for reviews. As a moderator, I have to read across the entire archive, in fandoms I might never otherwise go to on my own. I've found some wonderful things this way, and I'd encourage others to explore.
I will say, though, that even my boundless enthusiasm can take a hit when I'll read outside my areas of interest, leave reviews, and then hear an author I've reviewed remark that they don't read [insert fandom/genre of your choice here]. I write het, slash, and sometimes even SFW as the mood takes me. If you're going to ignore what I've written because you don't like het, or slash, or SFW because you can't muster the imagination to fap to it, then don't complain when you don't get reviews. I write a lot more original lately than fandom, so I won't even mess with treasured ships. Really, it makes me unlikely to continue to review those authors in what little free time I have.
Pay it up, people. For every review you get, leave two. In turn, those authors should go out and leave two more reviews each. In no time, we could have a lively community of engaged readers and authors, chatting happily in the forums about the reviews left in the archive. And we don't even charge a membership fee. Imagine that.