We've recently had a spate of questions on the users list regarding features such as rack-awareness, the trash can etc. which are not clearly documented from a user/admins perspective. There is some Javadoc present but most of the "documentation" exists either in JIRA or in the default config files themselves.
We should generate top down configuration and use guides for map/reduce and HDFS. These should probably be in forest and accessible from the project website (Javadoc isn't always approachable to our non-programmer audience). Committers should look for user documentation before accepting patches.