People often feel that wiki's are hopelessly unstructured and hard to navigate. However, there are some unique ways of working with wikis that once familarized with make them a lot more manageable.
Mediawiki provides a "Special" page called AllPages that let's you see an alphabetical listing of all pages in the wiki. After a certain number of pages (which our wiki has reached), it breaks the index up into different pieces (so you have to click twice to pick a range, which is kind of annoying, but it works). This page is so useful, I have placed a link in the Navigation section called "Index". (It's possible to add new links to the left bar by editing a page called MediaWiki:Sidebar).
Try it: Special:AllPages
Available Under the "Navigation" on the left, Recent Changes is often a very good place to start as it reveals where activity is taking place.
Try it: Special:RecentChanges
Mediawiki provides a number of "special" pages that give really useful views of the contents of the site. To see the full list of these "meta" pages -- "Special pages" under Toolbox.
Try it: Special:SpecialPages
Gallery of New Files
Another good, and more visual way, to see what's happening on the wiki.
Try it: Special:NewFiles
Often it's useful to know how a page or a file is linked / used. Media wiki provides a list of "backlinks" via the toolbox link "What links here". This is particularly useful for file views (such as those in the "Gallery of New Files" above -- as you can find the page that embeds a given image for instance via the "what links here".