The Zillow Real Estate Wiki is a good idea. But it's also an example of why a wiki might work best in a closed system. Zillow's intent is to create a comprehensive resource for home buyers by letting everyone contribute, but the result is more like a combination resource guide/marketing tool/crap sandbox. They need professional, dedicated editors.
Looking through the A-Z list there are many entries that are simply place names, for instance "Illinois." Click on it and at present you get nothing. I'm not sure what I'd expect to find by clicking on "Illinois" but I would expect to find something. Someone obviously created an entry and then got bored or wasn't sure what to do next.
Another entry is called "Az Realtor." As you might guess, some realtor in Arizona decided to get some free marketing. Very altruistic of them.
So while I am a fan of wikis in general, I am also a fan of good editing and content approval. Wikipedia has a strong editorial component, which I've encountered when attempting to change my company's listing. It was annoying, but I ultimately respected the fact that the editor wanted to make sure everything was kosher. Zillow needs that kind of editorial commitment and then the might have something.