Math Educators Stack Exchange has been progressing far more rapidly than most proposals ever did, especially in definition and commitment, where each stage usually takes several months (this is probably one reason the public beta has been delayed, to allow people new to the system to become familiarized with the Stack Exchange system).
Even now, the number of users with good reputation is increasing far more swiftly that most new sites, as can be seen by browsing beta sites on area51.
So why is this site so popular?
My answer: The other mathematics sites are already popular, but they are necessarily fragmented by different subject areas. But mathematicians in every subject area can be experts in teaching. And beyond this, elementary and secondary education teachers add an entirely new element of growth to the Stack Exchange network.
There are many more expert math teachers than expert math researchers.