Jeez, this is a tough crowd! I've always loved Davidson's subway photos. He obviously wasn't the first person to take photographs on the subway, but his pictures are instantly recognizable -- the color, the flash, the wide angle lens, the framing, the choice of subject matter. That is not an easy thing to accomplish as a photographer. And he accomplished it without making his pictures gimmicky or bizarre. You never get the sense that he is indifferent or condescending to the people he's photographing. He isn't Martin Parr.
I also think it's easy in hindsight to pooh-pooh the difficulty of the project. Davidson wasn't taking a few pictures on the subway from time to time. He was riding around all day for weeks on end while lugging his camera equipment. That's how he captured so many interesting scenes. In the early 1980s, you too would have been apprehensive if, in essence, you worked full time in the subway system while carrying a load of heavy, expensive equipment.
On a final note, I had to chuckle at the idea that these photographs were taken with the mass market in mind. If only! Subway has been out of print for seven or eight years now and, as a result, the price of used copies of the book had skyrocketed. (I always regretted not buying a copy when it was last in print, which is why I'm so happy that the new edition has been published.) If Davidson took these pictures as a mass marketing scheme, he didn't follow through with the plan.