The Linux iSCSI project keeps its documentation in a world-editable wiki. I should really say kept, because it's hard to find much usable documentation in the wiki at the moment; most of the pages are overgrown with wiki spam. Some pages have had a thousand edits in two days, all of them spam. All of this makes the project's wiki an unfortunately excellent illustration of why most open wikis are doomed.

The problem is that there are just more spammers out there automating their attacks than most wikis have people to fix the damage. Wikipedia survives because it has a critical mass of people who look after it, but it's an exception; very few wikis attract that many people. With a critical mass, you can block spammers and fix spam damage fast enough to discourage spammers and keep your wiki attractive; without it, you drown under a slowly rising tide of spam (and there is some evidence that existing spam attracts more spammers).

(It's not enough to have some dedicated people; you need to have enough that none of them have to spend too much time tending the wiki. Cleaning out spammers is drudge-work, and too much drudge-work burns people out.)

It's possible that the iSCSI wiki was so significantly hit because it doesn't use rel="nofollow" on external links. On the other hand, there's a fair amount of evidence that spammers just don't care about that and will hit anything within reach. And open-edit wiki pages are eminently within reach.

I don't have any answers for how a new wiki is supposed to survive long enough to (potentially) get a critical mass of users, although I wish I did. I just know that I can't imagine running an open-edit wiki myself if I had any choice in the matter, and I continue to be glad I didn't try to build one.