I love Wikipedia. It's the original site that works because of the constraints it imposes. You have to follow the spelling and grammar. You have to format it this way. You have to use the objective voice. All the pages are going to look this way. Jack Dorsey mentions this same idea in the context of Twitter: its strength is the 140-character limit. The great thing about the constraint in the case of Twitter is that it guarantees you'll be able to read something quickly. And in the case of Wikipedia you can consume the information very efficiently because you know what to expect and your brain isn't trying to wrap itself around other fonts or formats.
Another way I look at Wikipedia is that it's partly a group of people that read the news and then turn it into history. Last week, Gabby Giffords and 18 or 19 other people were shot. Afterwards, a series of news stories about what happened came out, each one filling in more and more details, but then the next day's news comes out and that story falls out of the public eye. Wikipedia reattaches those bits of news not just to a date and time but to the relevant actors in history.
The community also does a good job of carving out the boundaries of what's notable so that the set of stays manageable. My interpretation is that the collective subconsciously chooses not to allow stuff that it can't take responsibility for maintaining.
For example, I once wrote a Wikipedia article about Chaz Clemons. He raced against my school in high school, won states, and went on to win four Division III National Championsips in in the 100-meter dash. There are sources for his career like his university and news articles. What I wrote was factually correct and cited sources that weren't questioned. But the article was deleted because the group of Wikipedians who maintains articles about track athletes in the United States determined that it didn't mean Wikipedia's notability guidelines. So, there are maybe a few hundred people who wonder what happened to him and can't find out on Wikipedia. The tradeoff they make is that now Chaz Clemons' buddies can't come along and mess up his article saying funny things about him. In the aggregate, deleting articles that won't be watched carefully is an important way that the encyclopedia maintains its consistently high quality.
A lot of times, too, people say to me, "As Quora gets bigger, isn't the quality of answers on the site going to degrade? Don't you know people on the Internet are stupid?" In the face of that, Wikipedia is inspiring. It's reassuring to be able to look at what you're doing and say, some percentage of people really care and are smart. If you can get them interested, they can build something of real value.