I really love Wikipedia.  I think the concept is a good one — allow users to add their expertise to content or create content themselves.  However, I think it is a bit too optimistic to expect that people who do not create accounts can be trusted.  Time after time, I have seen the damage to that site that is wreaked by vandals, and I think it takes up too much time on the part of other users and admins to clean it up.  I don’t think it would go away if users had to register to add content, but I think it would diminish.  Ultimately, there is very little we can do to keep folks from wrecking the web, I guess, but we can make a bit more difficult.