As an American, I think there's some truth in what you say (how's that for hedging). Not sure if it's a general rule but Americans tend to be very public in voicing their opinions. Could argue that we're also less empathetic, or at least often less civil, too.
Thinking about it, do you find that American's (as opposed to other nationalities) tend to be more black/white than "shades of gray"? That there is an underlying assumption that there has to be winner and a loser and the possibility of there being more than one "right answer" is considered vaguely un-American, resulting in less tolerance for others views. Or is it more relate to what topic is being discussed?
Do you find that Americans are quicker to assume that an inadvertent insult is deliberate than those from other countries? On the internet, at least, people seem more likely to assume insult than the contrary, without regard to nationality (and Mudcat isn't the worst by far).. Are American's better, worse or simply average?
I've had major virtual fights with people on forums, met them later and wondered how I could ever have thought they meant to be insulting. And met people I thought were reasonable and rationale and realized they weren't at all.
A final question: TV often seems to portray American as verbally aggressive and less "civil" in general. "Meeting" someone on a forum, I wonder how much of the perception of what someone (from America ) "really" meant is colored by that? (When in doubt, blame the media. Or too much beer).
As a confirmed cynic, this may be too jaundiced a view. I'd be interested in your's and others comments. (But don't want to invite nationality bashing - probably a futile wish).