There is a rather unfair stereotype of "American travellers = rude". I think it needs to be dispensed with right away. All the Americans I've met while travelling were super friendly, pleasant and polite.
I have to wonder if it's just because we see more tourists from the US than any other place, so are exposed to more bad ones through basic probabilities. I mean, I may run into 25 tourists from the states while wandering round the coliseum, and 3 of them may be rude/loud/annoying. But I'll only run into 2 people from Norway. Given the population of various countries and if they had a set percentage of travellers coming out of them (although I'm guessing they don't) I'm just more likely to run into more annoying Americans than annoying Norwegians, even if, say, 10% * of the population of those countries are naturally rude and annoying people
* numbers totally made up
Add to that some general cultural differences, what with some cultures being more reserved than others in general, and it might be harder to spot, say, a rude Finn for example, because they may be less likely to strike up a conversation with a stranger that exposes that rudeness to the world.
Just my 2c from my observations travelling around, and meeting rude and lovely people alike from many countries