Here's another aspect on who should, or shouldn't, sit in the exit row:

On a flight last week I was asked by a very petite woman to help her with her carry-on. It was too heavy for her to lift and put in the overhead bin. If she can't lift her own bag, how is she going to handle the plug door in case of an emergency? The situation kind of solved itself when an able-looking passenger showed up and took the middle seat.

On the return trip, an obese passenger was asked to move from an exit-row seat, but shouldn't the first passenger have been asked as well?