I would honestly just recommend people take a few psychology courses, or read up various theories of psychology. Even though some would suggest that there is a culture hegemony here, my experience is that we tend to be so diverse that one source of explanation will never be credible. 

I would never ask my overseas friends about their country with the assumption that they are an authority of all regional and cultural aspects within their country, though they may be aware of them. I can learn about people better from learning about behavior in general due to the universality of much of it. 

I've wondered, how generally nice do you see the avarage person as? In Canada, one usually holds the door for others, especially if they're carrying things, and if you're walking behind them when they drop something without noticing, you pick it up for them. And when you talk to strangers, you smile.

I think I mentioned to you before, I've never interacted with an American I disliked, and that's still true. Still, my sample size is not that large. Would you say the general public usually shows common courtesy to one another? I know I've met my fair share of Canadian jerks.

Most of the time everyone is pretty polite in my experience, there are some rude/creepy people, but every country has some people like that

Well I was more speaking of more specific cultural things rather than general etiquette but I would say yes, people hold doors and cars wait for people to pass. I mean not to say everyone is always happy and polite but generally anywhere in the world people are more likely to be nice if you are nice to them.

One American custom I'm given to understand is not as common in Europe (and perhaps other North or South American nations but I don't have as much evidence to corroborate) is the practice of mandatory tipping. Not every state in the union requires the tip for the server as part of the restaurant bill but even the ones that don't, Americans almost always leave a generous tip. Hell people leave tips even for not particularly exceptional service. Not every American tips the waiter/waitress or delivery guy but to not do so carries a big social stigma as it's seen as stingy and cheap. The person usually is called upon to justify why they don't do it. See the opening scene of Quentin Tarantino's Reservoir Dogs.

(Love Canada and its people though. One auto shop, I THINK was in Repentigny, stayed open to help my family when our car broke down on our way to Mt. Tremblant.)