Documentation is always the worst part of any software development project.  The Agile guys believe that code should speak for itself.  While that's enough for many people and enough for many snippets of code, it's not always quite enough for all of the right people.

I believe firmly that the ease at which a new person can approach a piece of software demonstrates the community's overall maturity.  The Java community (in aggregate) has an impressive amount of documentation on its libraries.  The creation and standardization of JavaDoc led to much of that, but it still takes a bit of effort from the developers and the community.