That's a tough definition. I don't think many of us credit our non-original ideas. I don't even remember where I first read most of the stuff I know.
Am I mistaken, or has the exact post in question been made before on FR? I have a distinct visual memory of a post made a year or so ago that was several paragraphs long, mostly a quotation, but had an odd sentence or two added in.
An excellent question. It looked strangely familiar to me, which is why I initiated a Google search which led to the discovery of the referenced URL.
As far as having been used on FR before, I can't be sure; if it had, it should show up in a google search. It didn't on the phrase that I searched last night, but it might on others.
There's a bit of a gray area in between ideas that need citations and background knowledge that "everyone knows". As a general rule, the more unique, novel, or specific the idea is, the more likely it is to require citation if it's not your own original thought.