How does Stack Overflow compare to Yahoo! Answers, Experts-Exchange, ProgrammingTalk etc.? I want to know the critics! Why is Stack Overflow the best? What is the reasoning behind it? Is it a secret? What is the patent number? Why isn't it open source?
Joel Spolsky has said on the podcast (sorry don't feel like looking up references) that the main inspiration to create this site was the crappiness of Experts-Exchange: the way they came up on Google results but when you clicked through you didn't actually get any results. The other major inspiration is that so often a Google search would lead to outdated information on message boards. On Stack Overflow, the idea is that the voting mechanism lets the best answers always be on top. And everything is editable in a wiki-like fashion so that outdated information can be updated. Also, the creative commons license was inspired by sites/projects like experts exchange, cddb, imdb, etc, that started off as collaborative, community-generated content but then all of a sudden someone owned it all. With the creative commons license, SO doesn't own all the user-generated content on the site, and the complete data is available in monthly data dumps.
Here's a good video where Joel goes over a lot of this while talking to developers at Google: http://www.youtube.com/watch?v=NWHfY_lvKIQ
I'm not going to repeat myself from this answer, so I'll just quote myself :)
Please ignore the sarcasm. The original person was trying to get an Open source version of SO and I really get annoyed at those people. The dev team put hard work into the product, and they should reap the rewards.
The software behind StackOverflow is not open source. Many people don't like that. This is because everyone on the internet expects things to be free. Unfortunately, Jeff and Joel buy into the idea that when you put effort into making something that gets really popular, you deserve to eat, sleep in a house, and generally be able to make a living.
Of course, many people do not subscribe to this theory, and they have attempted to clone StackOverflow. Some people claim it can be done overnight, and some people have actually done things that are similar to StackOverflow. However, they do not have the polish or the community that StackOverflow has, so they are not really that great. You can read Jeff's post on this subject, and you will quickly realize why these don't work.