The Mercedes statement is a good example of one that would be taken as a boast by most people who drive non-status-symbol cars. People don't ordinarily inject info on the make of car into such a statement. The needless and oddly placed info, plus the status symbol, equals a likely and reasonable perception of an intent to brag that one has a Mercedes. One might as well mention the leather seats and premium stereo too.
The thing revealed about an offended parent of an ordinary child, upon a mention of giftedness of another's child, is primarily that the offendee values intelligence, and doesn't take pleasure in other people openly asserting the superiority of the others' children in this way. Not necessarily a gray area, just basic human nature of many parents.
Now, it might seem to be mostly in the eye of the beholder when the speaker doesn't know the gifted status of the listener's child. As parents of gifted kids we'd love to mention things about our children just as parents of ND kids do, and a normal part of that for us might involve mention of giftedness, or making it obvious. Still, due to the rareness of giftedness, it's highly likely that the other parent will have a child that's non-gifted. With exceptionally and profoundly gifted children, the likelihood approaches near certainty that the other's child will be lower in intelligence and/or achievement. Under those circumstances, whether it says something or not about the other parent, be prepared for an innocent drop-in of a key detail to be taken as bragging.
