Gelman on experiments as the gold standard

Andrew Gelman writes:

Experimental reasoning in social science:
In the present article, I'll address the following questions:
1. Why do I agree with the consensus characterization of randomized experimentation as a gold standard?
2. Given point 1 above, why does almost all my research use observational data?
Here's the full article, which is appearing in a volume, Field Experiments and Their Critics, edited by Dawn Teele.