Naiveté, when it comes to trusting the government or media. Several generations ago people for the most part automatically believed what was in the newspaper or what the government told them. There were no reports of child abuse in schools or churches, drug use, teen pregnancies, gays and lesbians, or corrupt police officers. We went to bed believing America was the center of the universe and the rest of the world was a "vacation destination". What happens with them has no effect on us.
The truth is oftentimes very unsettling to a lot of people and ignorance really is bliss. Not much is really "new". It's just out in the open more and the numbers are higher. I would never say having more "awareness" of how the world (really) works is a loss.
Having said that if I had to pinpoint anything I think we lost it would be (respect), especially that given from children to adults. Maybe that began when adults started introducing their kids to other adults using their first name instead of Mr. or Ms. (Back in my day there were no 5 year old children calling any adult by their first name). It's been a slippery slope ever since. The Mr. and Ms. served as a built in buffer as to how children and teens related to adults. One man's opinion! :-)