I disagree with average. Story ain't headed for a gold glove, but man oh man does he do a lot of things well. Put it this way: maybe his physical skills are average, but his intellect/instincts are way above average and that is crucial for all shortstops. Plus I think both he and Bregman make consistently good throws.
The Sox, at least during the highly successful JH era, have tended to prefer hitting over fielding in their SS's. Their best fielding SS's don't last too long, but the ones who can hit do. This year Story is an asset on the field and at bat and the first Sox SS to do so in the JH era.