I agree with Jim, something is wrong, I see our developers community isn't growing and getting older. There is no formal problem to start contribute, but steep learning curve and lack of mentoring practice scare people.
I agree, that learning curve is very steep.
Back in 1999 there was Programmer's Guide (edited by Thomas Lockhart). Sad, that this part of the documentation is not evolving anymore — it'd be just great to get a high-level overview of, say, query parsing and planning, or details of libpq protocol. Perhaps, this is a good project for a newby to do. Perhaps, it is worthwhile to create Developer's documentation, either as a section in the official docs or as a separate resource with structure and design similar to the official docs?
Another very wanted change in the community is mentorship. Personally, I don't feel confident to ask endless questions I have when looking into the code, as I understand, that this might be a very basic (for PostgreSQL hackers) stuff. For me it'd be a great helper, if I could talk this over (via e-mail or any messenger) with experienced developer. Reminds me of what we do for the GSoC, where developers volunteer for mentoring students.
Something similar would be handy in general, perhaps with a web interface similar to the CommitFest's one.