Interesting thread.
You can't always chuck general rudeness/impoliteness up to just cultural differences. The reality is that some places are just really unfriendly on the surface. But, that said, there are always dear people everywhere in the world (hope so!).
Sorry, but I found Western Europe to be very rude in general when living there, particulary the big cities. I think hospitality is just dead there, unless you're filthy rich. Again, there are some friendly folks here and there, as there is everywhere.
Australians are no conservationalists (just mainly small talk), but they are rather friendly on the surface and laid-backed (lived there 2 years).
The Far East in general has great hospitality and very polite people, particularly Japan and Thailand. Also, I hear China is pretty darn nice as well in that department.
The vast majority of Southern US is EXTREMELY hospitable and friendly, even though there are things I don't like about it.
I don't find Montréal or Toronto very friendly at all.
Rudest places I've been in the US: Boston and Washington D.C.
I've heard some nasty things about the SF and Los Angeles, but have never been there.