A year ago I asked a question on stackoverflow as I was having problems with postgres, since then the question has had over 15,000 views.
If someone would be willing to write up a definitive guide explaining core concepts of why things go bad and how to diagnose these issues yourself I'm sure it will get lots of views.
http://stackoverflow.com/questions/12472988/postgres-could-not-connect-to-server-no-such-file-or-directory
[edit] Here is a more pithy discussion on the problem: http://blog.ringerc.id.au/2012/09/postgresql-packaging-on-mac-os-x-is-mess.html?m=1