I imagine my primary failing, then, is that I assume people who are not described in a criticism will not take offense. That is, those who are actually trying would be as pissed off at those who are bitching instead of helping that they would understand what I’m getting at.
How the heck do I voice that without being so simplistic as saying “if this doesn’t apply to you...” or “you know who you are?”
There is a big enough difference between saying "some of you" (caution) and "most of you" (danger) but when you turn it into "all of you," watch out. No one likes to be painted with a broad brush, accused of something and found guilty for no reason.
"One or two FReepers are loony."
"A few FReepers are loony."
"Some FReepers are loony."
"Most FReepers are loony."
"All FReepers are loony."
See the difference? There are some who wouldn't even like the first sentence written above because we're all partners in a common cause. The more generalized you get, the more danger there is.
I imagine my primary failing, then, is that I assume people who are not described in a criticism will not take offense.
But that's not what you did. You described all Americans as being a certain way, and that's where the lack of communication came in.