Well this issue really isn't just one for the forum but probably one that linguists and proof-readers must also struggle with. Isn't there some advanced software that academic institutions use to detect plagiarism? Wonder if we can get an overview of the algorithm used by that software. That could easily govern the set of rules that should form plagiarism.

One more thing to take note is that most users who get onto good campaigns start getting scrutinized by a lot of people. I remember that there was an instance of plagiarism even against someone like "Lauda", who i believe was never at a loss of words and would have been silly to use someone else's words.

Then again, writing 2000 plus posts over 4 years while reading articles and gathering information from multiple sources, is it possible that a person's words may seem plagiarized. I think it is difficult to come to a conclusion without some real world exercise. Why not do a challenge here at the forum. I think there are a lot of original posters here with 2-4000 samples of their writing available. I can emphatically claim that I have NEVER copied anything verbatim or intentionally. If i use an article, i just put them in quotes. Yet, who is to say that I never missed adding the quotes in making 2000 posts, or that my thought train was not somehow affected by what I may have read somewhere else, in a way that even if am writing those words on my own, they are sub-consciously mirroring something I have read the same day or a few days before.

This could be an interesting exercise if done on the, say, 100 most prolific and discernibly original posters on the forum. I think I will make the cut in atleast the top 200, if not 200. Anyways, that an idea right there for the OP to check "Where to draw the line".