http://www.paulgraham.com/america.html
He actually has several essays on how to try to create a Silicon Valley in other geographic areas, but this seemed the most relevant.
tldr:
1. The US Allows Immigration.
2. The US Is a Rich Country.
3. The US Is Not (Yet) a Police State.
4. American Universities Are Better.
5. You Can Fire People in America.
6. In America Work Is Less Identified with Employment.
7. America Is Not Too Fussy. (If there are any laws regulating businesses, you can assume larval startups will break most of them, because they don't know what the laws are and don't have time to find out.)
8. America Has a Large Domestic Market.
9. America Has Venture Funding.
10. America Has Dynamic Typing for Careers. (Compared to other industrialized countries the US is disorganized about routing people into careers. For example, in America people often don't decide to go to medical school till they've finished college. In Europe they generally decide in high school.)
11. Attitude (There's one item conspicuously missing from this list: American attitudes. Americans are said to be more entrepreneurial, and less afraid of risk. But America has no monopoly on this. Indians and Chinese seem plenty entrepreneurial, perhaps more than Americans. Some say Europeans are less energetic, but I don't believe it. I think the problem with Europe is not that they lack balls, but that they lack examples.)