My guess is that Amazon will want to effectively "own" whatever city it selects. Not only will they get incentives from government, but they'll want to have primary influence over the business and educational communities. So that when Amazon wants something, the city will go along. This will give them a laboratory for all sorts of experiments, which is both fascinating and scary.

While a city like my hometown Atlanta makes sense on paper - transportation, great schools, awesome cost of living - there are too many other big players here. I think they want a next-generation "company town."