I want to make use of a wiki for two different reasons.
The first is for internal project management. Developers would be able to track issues, discuss and comment on them. Ideally the client would have some access to information which is relevant for them.
The second use is more for a general place where info can be updated and accessed. It would be more like a companion for my blog where I could start to build a useful database of info related to the blog and probably also eventually provide information about product usage (when I get those to market).
For the first use I will probably use Trac which will allow me to integrate with some of my engineering tools (like source code revisioning). Because it would only be accessible to a limited set of people it would be fairly easy to manage.
The second use creates a few more challenges. Have a publicly accessible and changeable wiki means managing both spam and content. The choice here would be more reliant on user accessibility and management features. Mediawiki (as used on Wikipedia and The Forum SA's wiki) may be a good choice just because it is widely used.
So what things need to be taken into consideration? Which options have good SEO features? Which ones have good SPAM management features? Which are the most usable?