I've always assumed the exit row "responsibilities" were a joke due to the people I've seen sitting in the exit row and allowed to sit there by the FA. It just seems to me that it can't be a hard task at all.

I had an unusual experience on a Delta flight though that changed my mind a bit. I was seated in the aisle seat immediately behind an exit row. My seat was not ticketed as an exit row. The FA did the speech and made everyone in the exit row to confirm that they could perform those tasks. She then looked directly at me, and said "you too sir, I need you to say yes as well". She didn't ask anyone else in my row to say yes.

If those doors are really in the range of 50 to 70 lbs, and people are paying extra to sit in those exit rows, it seems like the airlines are putting themselves in a bad position if someone is unlikely to be able to handle that but paid to sit there.