For hundreds of years ago I think perhaps evolution would be the toughest thing to explain. That and human rights - a black president? really? Women can vote? Ok but they can't own land right?
For 100 years ago, I think it would be the internet. Also it would be very difficult to explain the insane wealth that we all have to someone in 1913. We have food coming out of our ears. Everyone has multiple cars, heat, access to soft comfortable lightweight clothing that we constantly wash and throw away. We can afford to travel anywhere in the world at a moment's notice, we have massive homes, fine furniture, ridiculously comfortable mattresses, etc. etc.
For 50 years ago, I think it would be the fact that we're still using internal combustion engines. Seriously, where are the flying cars?