I have created a first draft of items that I think should (or might) be in such an initial introduction to wikis.
Due to the restricted formatting options here (and because I hope for contributions), I've chosen to publish it as a wiki page: http://scratchpad.wikia.com/wiki/Wiki_sales_pitch
This page has become quite a nice collection of thoughts on the topic by now. Thanks to everyone for their contributions!
The only thing that bugs me now is that it's still located in the Wikia Scratchpad; after all, the quality of that article has certainly passed the scratchpad phase by now (if a wiki page ever does, that is). Is there something more permanent (or prominent) we could move this to?
-- F.