All of this sounds like "The decline of Wikipedia". Number of edits and active users keeps falling. Many people tried to find the reason, but maybe it is just natural? It is no longer peak popularity for either page. Also there is always some churn on science/tech related "forums" - same questions/problems are posted over and over - not many people can stand it over long periods of time.
It's certainly the same phenomenon as Wikipedia -- certain users become influential, and some of them use their influence to solidify the way they do things and make the site more like what they want. This often is at odds with what a new user wants because they can't remember being a new user.
SO's many levels of reputation that give you additional abilities, I think, made the whole process happen faster. It got the site ramped up and turned into a reliable resource faster, and it also let people create their fiefdoms of influence where they wall out new users faster.
