Where MediaWiki shines is in its scale - because it is used so heavily by so many people, including some very high profile applications, it tends to have everything you want either built-in or as an extension. There's a rich set of special pages for sight management, and it's been pretty easy to stop spamming with minimal effort. The template mechanism makes creating consistent in-content markers (e.g. standard disclaimers and boilerplate) really easy.
Here are a few of my favorite extensions:
- FCKeditor. This is the must-have extension. It provides full rich-text WYSIWYG editing to the text editor. The transfer between Wiki and the rich preview isn't quite perfect, but it is very, very close. The editor has hooks for useful Wiki functions, like adding categories, templates, and browsing uploaded images.
- EmailAddressImage. Makes an image out of email addresses, to prevent email scraping.
- SpamBlackList. Honestly I'm not sure how well it's doing - we get very little spam.
- NiceCategoryList. Automatically generates topic-subtopic charts that are a bit nicer than what is built into MediaWiki.
- ImageLink. Provides a lot more flexibility in embedding images. Since I like to use images as "big icons" for site navigation, ImageLink lets me do within a Wiki page what I would normally do via HTML/CSS.