Alec Soth, on his Little Brown Mushroom blog, pretty much nails it here, regarding Robert Frank:
#14) As usual, when I read about literature, I think about photography. More often than not this gets me to thinking about Robert Frank. What makes The Americans so great, I think, is that it is the work of a profoundly introspective artist looking outward. The scales are balanced. While I love some of Frank’s later work, much of it looks like it was made by a sixteen year old emo kid drowning in introspection. I wish he’d spent more time after The Americans pushing back out into the world.