I think we can trace the entire decline to OSA's departure!
Just kidding. I think the disappearance of the search function, influx of spammers, and rampant d-dubbing (deleting all one's posts) make the site much less friendly to newcomers. Some have suggested that cliquishness also plays a role; I'm not so sure. BLSD and the on-topic areas don't seem too cliquish to me (and if they are, they were much worse back in the day).
ETA: Hi Alci!
ETA2: Also, the facts that the site breaks down and there are lots of database errors (e.g., posts not showing up as new, posts not showing up at all until you are in the "reply" view) don't help.