What REALLY sets Stack Overflow apart? Why do we love it? Why does it have more hits and posts per day than any other programming web site out there, when it's barely more than a year old?
Well, first of all, it's arranged differently. "Traditional" Q & A sites are forum based, you ask a question in the Java forum or the C# forum, whatever it applies to. Stack Overflow uses tags, which makes it infinitely more flexible. Have a question on C# vs Java? What forum would you post that in? On Stack Overflow, it's easy, just tag it "java" and "c#".
Normally new forums have to created by the site admins to discuss new topics, and there are always the "Other languages that we didn't make forums for" forum. On Stack Overflow, it's extremely easy to create new tags, you "move" questions by retagging them, and it's much easier to figure out "where" to ask a question. Oh, and instead of having to click on all those different forums, navigation is easier because all the questions are asked in the same place. See this for more on tags vs forums.
Second of all, Stack Overflow is run by the community. The more you contribute, the more you can do. Most other sites have a dozen or so admin appointed moderators, while Stack Overflow has over 180 "moderators" (users with 10000+ reputation). This really gives me the feeling I can make a different and help out in the community.
Stack Overflow also just plain moves faster. Our users answer question quicker than any other site out there, and we're seeing new features added all the time.
Stack Overflow is truly amazing, it has blown away the competition already, and has nowhere to go but up.
Please add you own favorite features to this list, and thank to Stack Overflow's creators for giving us the best programming site on the web. ;D