Walrus -

"It always struck me that those Americans I had experience of (not an overly large sample, but varied), seemed to go from very mild "non-oatths" to "God-damned" then straight to variants on "F*ck", there seems to be very little between."

Can you expand on this? (Polite listeners can avert their ears). I'm Canadian, but I suspect the phenomenon you're talking about applies here - but I'm not quite sure what these more subtle "gradations in swearing" are ... other than where I grew up, there was a certain unspoken cultural/class marker: there were homes in which you could say "shit" but not "f-ck", and there were homes in which you could not say either.