Q: What would happen if the United States had a open-border policy on immigration? Pass a security check, register where you're going, and come on in. We continue citizenship constraints, so that it takes a long time to be eligible for government programs like welfare or to vote in elections, but other than that stay where you like as long as you like.
I think this would induce a massive push for economic and political reform around the world. Government power to exploit is derived from the immobility of its citizens. If the bridge club wants to impose an income tax on you, you can just quit the bridge club. If the federal government passes an income tax, it's comparatively harder for you to leave, especially if there no other countries accepting immigrants. (Hat Tip to Buchanan's Club Theory.)
Adopting this policy, I think there would be enormous pressure on other governments around the world to become "more like the U.S." with respect to political and economic freedoms in order to persuade the most skilled and beneficial citizens from leaving them for the U.S. This would be the most true for South and Central America, who have some existing institutions that can adopt change. Even if this did not happen and America just ended up with a much higher immigration rate, we would experience massive income growth as the number of brains in the economy increased. It probably would stimulate some favorable (though unpopular) institutional changes within the United States as well.
I think Matt didn't entirely agree, but I'll leave it to him to post in the comments. I anticipate some interesting comments on this one.