First, let's take a look at what it is they do that is so special. Basically, it is a way for people to ask questions, and have the community answer them. There is nothing new about this, in fact it has been going on (electronically) since the 1980's with Newsgroups on the Internet. When the World Wide Web hit in the early 1990's, Newsgroups morphed to become fora (forums). People post a question, and a discussion ensues.
What makes StackExchange so groundbreaking, is:
- Involvement from the community. Many people get instantly involved in the discussion, Q&A, to evolve or produce an answer.
- Credibility - StackExchange has developed a system of self-management, where users gain reputation for their various activities. If you ask a question that someone else votes up (likes), you get +5 reputation. If someone else dislikes it (votes down), you get -5 reputation. Same for your answers - likes and dislikes. For various other things you do, you also may earn badges. All add up to a reputation score, and as your reputation builds, you obtain more privileges.
- Self-Managing - the reputation score arises as a result of your interaction with the community. As you develop, you gain more privileges - you become able to help moderate. So as you earn badges for activity (my favorite badge is Necromancer - you answer a question that has been sitting around for more than 6 months), you gain the ability to review and approve other peoples' edits, make your own edits on other peoples' posts, and so on - all self-managed by other users with abilities similar to or above yours. The community wisdom emerges.
- Topics - StackExchange is divided up into communities (you can join multiple, and your reputation is separate in each). Communities are also websites - so StackOverflow deals with computer programming, SuperUser with all things computer (admin and usage), AskDifferent with all kinds of questions on Apple products, Academia for all types of professional academic topics, Android Enthusiasts...you get the point. There are literally hundreds, maybe thousands, of communities.
- Immediate Gratification - I have asked many different questions on many different topics, and almost always get an answer the same day. The communities are very active, and not trolled by people who just love to get angry at some perceived slight and go off on a rant. It works, and it works very well. In fact, anyone who does behave like that, I would imagine, would be losing lots of reputation.
Try it! Join any community you are interested in, and see how quickly it will become an invaluable resource.