yea you forgot the most important part - the two founders both had very popular blogs/followings.

seriously?

It also doesn't hurt that it was started by two individuals that were already huge in the DEV (esp. MS DEV) community...

I think one of the most important things was that Jeff Atwood and Joel Spolsky talked about it on their blogs, and those blogs happened to be two of the most popular programming oriented blogs. This got a lot of traffic and buy in from the programming community as they obviously felt the two had credibility and reasonable ideas.