My answer to this is:

The use of smart phones. 

To me, it's more about the frequency of use. It seems to become a rather normal walk of life to just walk around with your face glued to a smart device screen. 

I don't think it became strange to me until one day I just realized how often people are on their devices, and then I thought back to a time when those devices didn't exist..

So hands down, smart phones.