Class in open-source projects

(from a comment on Zen and the Art of Nonprofit Technology)

I've realized recently that open-source projects have developed a class structure. First, consider the broad middle-class of little-known but functional projects that have a number of users and several developers (Amanda, Buildbot, Cfengine, Sphinx, Curl, the autoconf archive, maybe GMT). Above these projects are the superprojects that nearly everyone uses and that have hundreds of contributors (Firefox, Linux, Python, Perl, to name a few). These are all islands in a squalid sea of abandonware and single-developer projects -- we've all got one or two, right?

What are the keys to mobility between these classes? What factors might keep an otherwise-deserving project in the lower or middle classes?