Stack Overflow was a progressive and natural evolution of the standard clunky forum.
Using ajax it created a more fun and clean user experience.
Using badges and karma to gain responsibility allow forums post to become a game. People naturally like to see progression and growth, being able to watch my karma go up was a rush, similar to a video game. (some people plan elaborate flights to rack up frequent flyer miles, planning a trip properly could cost as little as 50 dollars to fly around the world in 24 hours, and gaining thousands of miles using point multipliers)
Stack Overflow was search-able! Here is a thought, create a forum that is easily index by search engines… One question per page, on topic with the most relevant posts at the top of the page. Who cares about user bounce backs, if they see your URL enough (StackOverflow.com) they will end up creating an account and becoming a user who creates even more quality content!
The site was geared toward geeks, and programmers! They adopt new tech the quickest.
Did I miss any reasons? Feel free to comment below.
Update – Check out my community at four2go.gumyum.com
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