One element of this politeness is similar to the prohibition against lashon hara, or 'evil speech', in Judaism. As I understand lashon hara, people are expected not to say damaging/unkind things, even if true, unless that silence could harm someone else.
Of course, none of us are perfect... and not being Jewish I'd like to have some first-hand perspectives on what happens then. How commonly is the rule broken, and how seriously is it taken when that happens? For a middling offense, what's the social punishment likely to be? Do parents have any particular ways of raising their children to avoid it?