Why call this "defection"? I interpret "defection" as meaning not just "a bad thing people do" but as deliberately deviating from a previous agreement. The relationship between the prisoner's dilemma and not being sufficiently polite seems forced, or at least like it could have used more thorough explanation.

I agree with Alicorn and others who find the sort of forced extreme politeness of some of the suggested responses (especially to #1) off-putting. I can't quite explain why, but if I had to guess, it would be for two reasons. First, politeness level indicates status, and when someone uses excessive politeness that ascribes to me extremely high status that I don't feel I've earned, I suspect they're trying to manipulate me. Given that many of the arguments in this post are explicitly about politeness as a tool for manipulating people, this seems to be a valid suspicion.

Second, lack of politeness is a countersignalling method to indicate friendship and community by showing you are close enough to a person that politeness is unnecessary (consider the relatively common story of the friends who greet each other with racial slurs, like "Hey n*gga!", "Hey cracker!" partly as a bonding mechanism to show that they're close enough to allow what would normally be offensive). If everyone is on board with this, someone who goes around being extremely polite and telling others to be extremely polite might seem to be distancing eirself from the community and denying friendship.

I realize you don't intend polite comments that way, but as you say in this post, it doesn't matter what you intend so much as how other people interpret it. I'd agree that politeness is generally a good thing, but carpet-bombing all social interactions with politeness will tend to freak a lot of people out, and Less Wrong is probably more prone to that than other places.