Teejay,
I think your last paragraph summarizes the essence of your premise, and I agree with it. I think most of us try to look at the information presented and the internal logic that led to a certain conclusion. Although some may not be very far away from the WT days in terms of new information, I think they are very far away due to the fact that they have left. They are exercising critical thinking skills that were dormant as Witnesses, and I see that as a major difference. As a result, I think most of us are wary of being sucked in again by invalid ideas, whether they are presented here or else where.
Now here are my thoughts on source credibility. Your example of the WT may be true, i.e., they may have good articles here and there. I don't spend my time reading the WT because the rate of information return is low. Since they have routinely quoted scholars out of context, I don't trust them. Even when what they write appears credible, it is hard to verify it. They don't provide references where the context of quotes can be studied.
As far as posters on the board go, I don't expect a scientific treatise to appreciate the work. If the ideas are congruent with what I already know and the poster appears knowledgeable, that is generally satifactory to me. Now, there is no question, that I may develope a bias over time for or against certain posters based on their "track record", but I do try to evaluate each post independently of the previous ones. If I don't understand something, or I think the logic is unclear, I generally will post a question. If I disagree with a poster regardless of previous credibility, I will state that disagreement.
Of course, there are certain subjects where particular persons have unique credibility on a very specific subject. For example, Patriot, who is a member of the NYPD talked about his 16 hour days beginning with Tuesday's tragedy.
Frankly, it is this kind on uniqueness that the 2500+ posters bring to this board, that I find especially gratifying.