Katie Cunningham has written a great article entitled ‘Your wiki is a dump‘, outlining what goes wrong with wikis, and how to fix them.
I have seen this over and over again. Wikis need work, and they need someone with the guts (and/or the authority) to take a set of shears to them on a regular basis. If you have that, you can have a great resource. If not, you can have a nightmare.