The thing that I like the most about open sites like wikipedia is that it allows people to contribute. Content is controlled by users and this has made it an invaluable research site. Although I read it often, I never use it as reference.
Below is an example of how mischievous pranksters often hack into the system and vandalize its content. Although such offense can be reported, this reveals the apparent weakness of a free for all site.
I’m still a big fan of such sites (and softwares). The idea that its free and everyone can contribute to make it better is a great idea. But some people abuse the privilege. There will always be people like this in the world.
But in a way, such activities, evil as they may seem tells us how we can improve. And I hope that the Wiki people can increase security measures in the future.