As I ponder what makes various geeky media ventures succeed (I ponder a lot), I think one of the factors is what I've tentatively christen being "Socializable"
A "Socializable" media is something that draws people together in interests for the product. It may be something with a good universe that is "fanficcable". It may be something filled with witty quotes that spawn memes and interests people. It may be something that really speaks to people so they share it. But it seems a key to success is having some media product that enters into people's social lives and activities.
Consider Harry Potter, a novel that mixed a fun universe, odd characters (and names) that included in-jokes, and a very archetypical series of plots. Harry Potter was one of those lightning-in-a-bottle phenomena, but I also feel the property had a lot of "Socializable" components.
Look at video games of the past, where many mods kept people's interest, or games today with assorted content-sharing options. The games draw people in, tie them together in common interests and exchanges, and thus increase and maintain their popularity.
There needs to be something, to guarantee success, that adds a social factor to a media – that in short, makes it "Socializable", that means it draws people together. It plays on and plays into our basic social instincts somehow, be it sharing something neat, working together, being relatable, etc. We social creatures respond to those things quite well.
I'm not sure I can accurately define this "Socializable" trait – though much like "Geekonomy", I will probably try.