It occurred to me, after writing my first reply, that you might find it interesting to hear what my afore-mentioned ex-pat American partner has to say about the differences he has observed. I should note that he lived primarily in the north-east of the U.S., with some periods of time spent in the midwest, both north and south, and he's very left wing politically. Since coming to Canada, he's lived only in Toronto.

He told me that he finds it difficult to articulate the differences, but that they are nonetheless very evident. Some of the points he made:

-Canadians are less reverent toward their political leaders, public figures, icons, and their country, and are more able to make fun of all these things.
-Canadians are just as nationalistic, but don't take it as something deserving of high seriousness - they make fun of their flag, and so on.
-Canadians really are more polite; they also have a more highly developed sense of justice and egalitarianism, and pay more than lip service to ideas of equality
-Canadians are less prudish than Americans, but also less vulgar
-Canadians are less fearful - they don't have an obsession with guns and gated communities as necessary for personal protection, but it's not just a matter of being less afraid of crime, they're less afraid in general - fear is mmuch more of a general driving factor in American culture.
-"Canadians are heathens compared to Americans" - they are not any less spiritual, but are much less religious, and much less given to public displays of religiosity.
-Americans seem to have to make a production about what they do, they approach life as though they have to prove themselves all the time; Canadians don't.