And this is the crux of the biscuit.

A large measure of StackOverflow's success-- perhaps the most significant factor-- was the immediate traction that came from readers of Joel and Jeff's blogs. SO had a ready-made audience, due to the notability of its sponsors.

It's not at all clear to me that this is scalable via the Area51 model.

Clearly, it takes more than 360 people to make a site successful. The question is: who is in the best position to attract the attention of the prospective users quickly and effectively? Time will tell, I suppose.