[–] Herbert666Marcuse ago 

what traits made them insufferable?

The inability to have a simply pleasant relaxed conversation without interjecting snideness, sarcasm and snark.

The late GenXers & the Millennials don't know how different things used to be in this country, because the cultural poisoning of (((Seinfeld/Friends/SATC))) is all they've ever known.

And their Boomer parents were too d@mned asleep at the wheel to demand that their White children adhere to classical White standards of good manners, gracefulness and dignity during social situations.