You meet all sorts of unsavory individuals when you’ve been over here as long as I have and that includes student-loan dodgers. Just off the top of my head, there have been at least a couple that I knew, and I wouldn’t be surprised if there were more and they just kept quiet about it. But the ones I knew about, they never seemed to have a problem receiving services or obtaining a new passport at the embassy or even entering and leaving the US. I chalk it up to the behemoth bureaucracy that does not let the left hand of government know what the right hand is doing. Maybe some day the technology system of the US government will send a red flag to all official computers worldwide or even nationwide regarding this or that financial transgression, but it doesn’t seem to be the case yet as far as I can tell.
I don’t know about other countries, but I can tell you that in Thailand, I’ve never heard of a local company or school actually checking with someone’s school back home. If you have your diploma and maybe some transcripts obtained before you default and can no longer obtain them, that seems to be good enough for the Labor or Education Ministry.
Bankruptcy was mentioned somewhere above, and my understanding is that student loans are not covered by bankruptcy proceedings. You’ll still be saddled with those.
EDIT: I’ve never heard of any foreigner being refused a visa to Thailand for having debts of any kind. AFAIK, as long as you don’t appear in the criminal database of Interpol, you’re not going to be denied a visa to Thailand.