I am an avid user of wikis, and I am a wiki-wonderer visiting many different wiki communities (a GluePerson perhaps), and I am keen to promote the use of wikis for new internet communities, and also for corporate intranet use.
I work with web technology, and I have looked under the hood of the mediawiki codebase a few times. I've installed it at work, and on a USB key, and I've coded a few extensions.
...but enough about me. Here's my thoughts about MediaWiki:
I'm preaching to the converted when I say that MediaWiki is undoubtedly one of the best wiki engines, with a well honed balance between usability and power, but there is increasing competition. I'm not talking about the likes of MoinMoin or OddMuse (not as good as MediaWiki in my opinion). DukuWiki comes closer, but some "hosted" offerings like wikispaces.com and wetpaint.com seem to have the WYSIWYG thing nailed, and a commercial software called Confluence has some interesting features and nice design quirks which MediaWiki is missing.
But most of all MediaWiki is missing a decent marketing message, particulary when targeting corporate Intranet usage. This is a missed opportunity to push MediaWiki to a wider userbase, which will reap rewards in terms of more development contributions and attracting larger communties of wiki contributors to the wikimedia projects. A marketing message is something this mediawiki.org site should be tackling.
To help people establish a new wiki community (The Seed posting phase of the wiki lifecycle), I think mediawiki should come with some built-in basic user manual pages, and other meta information. Happily I found someone started this process, and I tried galvanise this into -a 'project' over at Public Domain Help Pages. This has been making some progress more recently (2007/2008). I'm "watching" that whole bunch of pages and ocassionaly expanding it.
I've encountered a lot of wiki spam during my wanderings. I think the Anti-spam features should be more built-in out-of-the-box, particularly to protect small wikis; Those wikis which do not have a very active community, and more importantly, those wikis which have a lazy administrator. Developers no doubt have little sympathy for such people (It is very easy to deploy anti-spam features) however we should be thinking about how to reduce the success rate of wiki spammers on a internet-wide scale. This might help to put them off (or target other types of website instead)