Someone in this thread mentioned the "Equivalents" of Steiglitz. This series was Steiglitz's reaction to a critique that he was capable of producing strong "people" photos only. So for the next several years he was the first to introduce abastract art in the photography domain by means of photographing clouds. But, he did not just turn his camera to the sky and started clicking. He was worried that the orthochromatic emulsions he used didn't reproduce exactly his vision (that's the technical part) and also that he wanted the poetic nature of the pictures to come through so that Bloch would exclaim "This is music!" (that's the artistic part).
So, I believe personal progress except tools mastery is also a product of the artist's reaction to sincere critique of his/her work. And by this I do not mean the comments below photos that show up on popular photo sites.
One should pick his critics as carefully as his photographs.