Every time I've tried to get non-developers to use a wiki, the difficulty of writing text has been the biggest barrier. A WYSIWYG editor along the lines of what StackOverflow has seems like a great idea to me. It still shows the markup in the box, so motivated users will eventually learn to use it (thus becoming more efficient), but it also shows a live preview, so that users get instant feedback as to whether their text looks correct.
Along these same lines, another annoyance is that every wiki seems to use slightly different markup. I guess each designer thinks they can do it better than the ones before. I would suggest using either MediaWiki's syntax, or something like MarkDown that's sort of standard, so that power users will have an easier time of things.
Finally, consider what about your wiki will set it apart from the many many many many other wikis already out there, and how your design will impact this.
