It's a bit of an exaggeration I think. Vivian Maier is tremendously talented to be sure, but "quite possibly the most important street photographer of the 20th century" is a preeeetty big leap. "Top street photographer of the 20th century" means you're going toe to toe with Henri Cartier-Bresson, Robert Frank, Elliott Erwitt, Daido Moriyama, Bruce Gilden, and a slew of others.
What does separate her from your average Flickr user is her technique and her knack for catching the Decisive Moment.
Go back and look at some of the pictures in the article. Now imagine having taken that photo 0.2 seconds later, or earlier. How many of them would've been as good? How many would be completely different and completely mundane? Knowing, seeing, and capturing the Decisive Moment is very, very hard, especially in the 50s and 60s before the advent of autofocus, auto-winding, and auto-everything.
She's also compositionally very competent. This is something that's harder to appreciate unless you're into photography yourself and have seen a large amount of work. Unlike most amateur street photographers, her photos are frequently not about the subject, but rather interesting light, textures, and geometry, in addition to the subject matter.
What does separate her from your average Flickr user is her technique and her knack for catching the Decisive Moment.
Go back and look at some of the pictures in the article. Now imagine having taken that photo 0.2 seconds later, or earlier. How many of them would've been as good? How many would be completely different and completely mundane? Knowing, seeing, and capturing the Decisive Moment is very, very hard, especially in the 50s and 60s before the advent of autofocus, auto-winding, and auto-everything.
She's also compositionally very competent. This is something that's harder to appreciate unless you're into photography yourself and have seen a large amount of work. Unlike most amateur street photographers, her photos are frequently not about the subject, but rather interesting light, textures, and geometry, in addition to the subject matter.