b) What's required in terms of basic structure?
That's a tough one.
That is what I see is the problem. Where I work small groups set up a wiki and learn to use the wiki as a group with few problems since the wiki admin is typically in the group. But when an enterprise-wide wiki is set up many come to it and have trouble determining first of all that it is a wiki and they can add content, then have trouble determining how to create a page, then how to place it within the wiki's structure. We use categories in one of our enterprise wikis and a Main Page that is nothing but instructions. I've concluded that designing a wiki is as much if not more complicated than designing a web page. But like a web page it needs to stand on its own and users will rely the Main Page to guide them.
The best way I've found for designing a useful wiki is to look at other wikis. Wikipedia is an awful example I think. But its so famous that people who want to add content are motivated to learn it. That is not the case in most offices. But there are many other wikis to look at and get ideas from. One of my favorites is www.cookbookwiki.com. They state at the top of the Main Page that it is a web page that you can edit. On the sidebar they have a "Resources" menu with links to setting up an account and creating content. If you follow the "Create Content" link there are instructions on what to do to create content, including searching existing content first so you don't duplicate. Its handholding but I think that is what needs to be designed into the wiki pages to guide the helpless new wiki users along and make adding content easier. Otherwise you will end up with a small number of contributors to your wiki and their content scattered randomly in the wiki, lots of orphan pages with duplicate content, and I have one of our earlier wikis to prove it.
So, in short, use other well designed wikis you find as guides and before releasing your wiki, have some novices try to use it. This user testing should show where problems are and where improvements can be made.
-Jim