"[Wiki] software is not magic," Wales said. "You can't throw up a wiki and hope a miracle will occur. It takes thoughtful people to make it run."
I keep seeing more and more of this as wikis become all the buzz. People throw up wikis at the drop of a hat with no clear purpose. If you are setting up a wiki, it should have a clear and defined purpose and serve a user community. A wiki just for a wiki's sake is a useless piece of software.
On the other end of the spectrum, I see many people misusing wikis. Building too much complexity, infrastructure, and workflow-like-concepts into wiki engines. The more complex a wiki system becomes, the less clearly defined its purpose is.
The most unsuccessfull wikis are the ones that are misused. Wikis are good at being wikis. You shouldn't try to use a wiki engine to create a presentation or as an IM client any more than you should use a wiki engine to iron your clothes. If you try either one of those, you'll be disappointed with the wiki software. And if you try steering your user community towards wiki-misuse, you'll find your user community confused and reluctant to use the wiki for what it is rightfully for.