Besides the community of engineers and scientists, what works well in Silicon Valley is a community of venture capitalists who have honed up skill in picking and funding technology with VC funds. For example, a story goes that Vinod Khosla listens to anyone in need of fund, not always to fund a new venture, but to be aware of what kind of ideas entrepreneurs are coming up with. Proximity between VC funding and the engineering community allows Silicon Valley to be a source of money needed to fund a startup. I am not sure where else is this happening. And I think this a big barrier that other cities will find hard to cross. There are tons of engineers with good idea, but there is not enough money to fund them all.