Techcrunch article: Wetpaint just launched (so that's why I hadn't heard of it). It's a wiki host and wiki framework rolled into one.

The interface battle is on: check out the awesome functionality on this history page -- each edit is programmatically summarized, for example ("4 words added, 2 words deleted"). (I'm jealous. Someone make me an OS X wikipedia client.)

Wetpaint is going to be pretty big. Bigger than Wikia, I think.

It's hard to overstate the importance of usability testing, which Wetpaint has obviously been through and wikipedia/mediawiki obviously hasn't. Not that the Wetpaint interface is ideal -- it's apparent the let's-maximize-revenue folks won out over the let's-make-the-perfect-zen-website folks -- but Wetpaint gets an important part right: in one fell swoop, thanks to ajax-powered editing, the mechancs of the site are manifest in its appearence: This Page Is Put Together By A Bunch Of People Like You. And a bunch of smaller confusions are nipped in the bud (users are called writers, not editors, for exmaple). (Meanwhile, I've watched web-designer friends click on the wrong edit-this-section link on wikipedia. A few developer-hours improving the link placement could save thousands of community-hours reverting edits by confused newbies.)

We geeks don't realize it, but wikis, unless specifically designed, are hard for most people (excluding teenagers, tech writers, etc.).

Sidenote: the story brought out a great comment -- on Digg, of all places -- analyzing why established wikis are less vulnerable:

I believe (and see it since I spend a lot of time on wikipedia for school) that most articles are "crystalized out" on wikipedia. I'm not saying wiki has stopped growing, but most pages on significant articles are finished, and remain untouched by ídiots [vandals] because there is no reason to edit it. You can expect that an article about Bush will be incorrectly edited a lot of course.

I think you can compare it to graffititags: people spray their logos only on walls, almost never on windows, doors, or higways. Why not? because they have a "usable" function.

That is why Wiki[pedia] works. it has its weaknesses, but it still is one of the strongest inventions on the web.