(TEST: Hmm - rubbish formatting here, what triggers noticing a new para ...)
The Gentoo wiki is very good as a MW site. The presentation is fantastic and very approachable - ie someone(s) really know their stuff about design. I run several MW sites myself and contribute in a very minor way to MW's docs and use several other people's/project's MWs, so I feel I have a fair grasp about wikis running on MW.
Each site has many templates, styles and so on that are idiosyncratic to that site. For example I created the original WIP template here and it has sprouted an author tag - cool: that's collaboration in process which is exactly what MW is all about. I was presented with a link to the template help - great: and I fixed my error.
However, most people are not particularly familiar with MW but may want to do docs because that is what they can do. These potential contributors may not be programmers or l33t sysadmins (well they are - they are Gentooers!) or whatever but can string one or two words together. These are the folk we need to enroll and encourage.
There is a link to the Help page on the front page but it is only a Category and a bit rubbish. The front page is very dense and needs weeding.
On my work wiki I put a bloody great link to a simple howto create and edit a page. I found that the shorter I made it the more people contributed. I gradually added links to more advanced stuff at the bottom as References - always in a new tab and people use them.
A handy experiment for user testing is to point, say, your (ideally non technical and non wiki focussed) partner at your wiki and ask them to contribute something with a few minimal pointers and see what they do. You will be amazed at how fast you realize how crap your carefully crafted wiki organisation really is.
I'll happily do the above - a simple set of editor docs - but it is a task that belongs to the owners of the site and I'm not one of those. I'll also drop that since I've added Visual Editor on my sites, I've seen a dramatic increase in contributions.