Documentation.

Far too often when I want to learn something new I either find there is not much documentation or find that there is a large mass of documentation that probably has everything I need but is so disorganized that I can't figure out how to approach it.

Another documentation problem, especially with open source projects, is that development and documentation are often loosely coupled, if at all. The people doing documentation usually don't have the resources to keep up with development, and so even if there is good well organized comprehensive documentation it is usually obsolete.