As Felten points out, there's something missing when the only alternatives are being forced to choose between the chaos of having to listen to everyone yammer loudly into their cell phones or being policed by those who angrily "shush" even the slightest peep in a "quiet car": Courtesy.
The phenomenon Felten diagnoses is the same one that has come to govern sexual contact between young people at politically correct places like universities. In part because of the erosion of universally-understood standards for proper behavior between the sexes (perhaps "chivalry" here serves as an analogue to "courtesy" or "civility"), the whole concept of "sexual harassment" came into being. And once that happened, "hypercorrectitude" took over, to such an absurd extent that, at some universities, specific verbal consent is required before each distinct act of a sexual nature that transpires between two people.
That's the real problem when civility and manners erode. The disappearance of more informal, self-governing ways of regulating human behavior gives rise to hard-and-fast rules and codes to be administered mercilessly, regardless of context.
David Brooks theorizes that the root cause of the decline of civility is a lack of modesty. And perhaps he's right -- for the first generation in which civility declines. Thereafter, sadly, courtesy or civility -- call it what you will -- continues to disappear apace because it's never been transmitted to or modeled for all too many young people.