I'm no expert but I believe the sulfur content is higher in North American diesel, but that's not too hard to handle, I think that's the old excuse and there are fairly clean ultra low sulfur diesels you can now buy. To pass California emissions, you need to put some sort of aqueous urea tank in to the exhaust system like an AdBlue or BlueTec and those cost a couple thousand dollars (I want to say it's like $3000 on a diesel sedan like a Subaru or the 320D)

I'm not sure why the costs are so high, it may be a patented technology, there isn't anything terribly exotic in it though.

If you can legitimately get in the 40-45mpg range, it's still a pretty good deal. A Lexus RX hybrid still only can get like 30mpg.

We didn't get ultra low sulfur diesel here until a few years ago: http://www.greencarcongress.com/2006/06/ultra_lowsulfur.html

Because there's a negative perception hump to get past in the usa. I tried suggesting a diesel, and everyone complains about how dirty it is and other myths.

There are diesels that get even better millage, like the Audi A2 with a mileage on it's diesel model ranging from 80 to 120 mpg. I never understand why car manufacturers don't make aerodynamic body designs for their cars. A honda insight with a diesel engine gets similar mileage.

It used to be the case that US diesel was of inferior quality to European diesel, but I’ve no idea if that’s still the case.

At least where I live (NH), diesel is more expensive than gasoline due to higher taxes.