One of the themes I've thought about is the critical mass required to sustain an online project. I was recently looking at some Wikia wikis and noticed they have few if any editors. And of course there are the endless debates on what it means when Wikipedia loses editors and admins because of drama, etc.
I think we can all agree that Wikipedia does have the critical mass of editors required to operate. It is creating loads of new content, refining it, having internal debates, and tackling the various backlogs in a reasonable time with regard to its size.
Looking at Commons though, I think you see what happens when a project is below critical mass. While Wikipedia attracts individuals with an interest in any topic you can write an article on, Commons mainly attracts those interested in photography and/or visual imagery. So it already has a smaller pool of potential editors to draw from. Compare [[WP:AFD]] and [[COM:DR]] and you can see that Commons is below the critical mass of editors to compete and beat other image services (compare the billions of images on Flickr with the millions on Commons and the tens of thousands of articles in Britannica with the millions in Wikipedia). I think more people would start with Google Image or Flickr when searching for a random image than Commons. Granted this could change at more search engines provide search criteria that include CC searches (Yahoo just started).
Lastly, looking at Meta, you can see a project that is almost exactly at or slightly below critical mass. Meta can create new pages, process requests, etc, but it lacks the editorship to update all the help pages, document the new features, update the lists of things on various wikis, etc. I remember a few months ago seeing a Help page as Meta that talked about how Usermod was still used for the Finnish Wikipedia, as Usermod was better for small websites.
My thought is something along the lines of how much above critical mass is Wikipedia? Wikipedia operates well enough that most people consider it an acceptable source for looking up basic facts in an informal environment. The average figure I've come up with is 205,000 edits a day to Wikipedia. Also, I calculated that since Jan 2006, the average daily edit count has ranged from about 140,000 to 280,000. So what is the critical mass of editors and edits to maintain it's market position?
We have about 10,000 active editors, about 900 of whom are admins. How low can those numbers go (and the corresponding daily edit count) before the public senses a decline in the quality (however poor it is now, it can get worse) of information provided? How low can that quality go before the public finds other sources of information that it finds better than Wikipedia? Right now WP is the 800 lb gorilla in the room. The barrier to entry for a new 'pedia is so high because Wikipedia's 200,000 edits a day is more than any new endeavor could ever match (look at Veropedia, Conservapedia, etc). How low could that number go before other entrepreneurs can create products that can compete with a smaller WP?