This box-of-four makes it easy to sort out who says what on a blog. If they agree with you AND are rational, they are "friends." If they agree with you but commit some crime against reason, they are "embarrassing allies." If they disagree with you and don't make sense, they're "crackpots." And if they disagree with you yet are sensible, they are "worthy opponents."
I love this little grid, although I don't view the "worthy opponents" category quite the way Sean does. It's the people who don't agree with me who do make sense who are most likely to teach me something I don't know.
That reminds me of one of the standard refrains from our homeschool years. When a child would make a mistake on math or some other subject, they tended to be discouraged by their error. I would always say, "This is an exciting moment! LEARNING is about to occur!" You can't learn what you already know, and you can't learn until you realize there's something you don't know already. It takes a mistake (or a disagreement) to get to the place where more knowledge can appear.
So--I think I might change Sean's grid to read "Potential teachers" where it now says "Worthy opponents."