I'm going to nitpick on this, but there wasn't free movement across the U.S. borders even before 9/11. The closest was the Canada-U.S. border, but even that required two pieces of ID, one with a photo (which has subsequently been changed to a passport for air travel and will soon be a passport for ground crossings). Entry from other nations generally required the traditional passport and sometimes a visa — I wouldn't classify that as free movement.
Now, one can make the point that the U.S. did not do enough to check up on those who entered the U.S. on visas, but's different from saying there was free movement across the U.S. borders.
Well, one good nitpick deserves another.
So, I'm going to nitpick and say that while, on paper, two pieces of identification may have been required to cross the U.S./Canada border prior to 9/11, the reality is that this was not necessarily enforced unless someone set off internal alarm bells for a particular border guard on duty.
I have crossed the border by road hundreds of times pre-911 and scores of times post-9/11 without having my identification checked closely, if at all. (If they'd actually checked, they would have noticed that my driver's licence bears a different name than my birth certificate and a different name than my passport, for instance, and they might have asked a couple of questions). I think it's all about the presentation. I always offer my I.D. before they even ask for it, and because I don't look like a threat, they very rarely looked at it in any detail, if at all. Several times, they didn't even take it from my hand and didn't even read it.
Don't get me wrong - I'm glad that the border lines are not tied up unnecessarily, I have no problem with the requirement for appropriate identification, and I have no problem with the upcoming passport requirement, but I don't think it is accurate to say that border crossings are or were policed and checked as closely as the regulations purport to require.
There has long been a relatively free flow of goods and people between Canada and the U.S., for good reason as it is mutually beneficial. That said, there is also good reason for taking additional precautions when it comes to the flow of goods or people from countries other than the U.S. and Canada across the U.S./Canada border (e.g. your visa point).
I would like to see the U.S. and Canada achieve an appropriate balance between necessary security and the free flow of trade for both countries that also does not result in a big honking lineup at the border (which might inconvenience me and other individuals personally, as well as interfere in international trade). The U.S. and Canada have been doing so fairly well for the past several decades, so I am optimistic that they will continue to do so.