The quality of documentation is something that I am sure many people would like to compare on.  A poorly documented wiki system wastes peoples time, and doesn't provide a stepping stone to learning more.  Some great wikis have terrible documentation.

Great documentation takes newbies through the whole installation process including installing web server and database.  Takes a howto achieve task approach.  Keeps the learning curve shallow.

Good documentation points to sources of information that the documentor hasn't got time to write specifically.  Learning curve might be steep, but isn't unmanageable.

Terrible documentation assumes the reader wrote the wiki system and just needs reminding what they called the functions.  The learning curve is a brick wall on which the documentor is sat on top, laughing at the poor fools who dare to think they can master their system.