I don't know about characterizing it as a profound variation in literary progress. Maybe, but yes, it is probably too early to tell, as literature, like history, does require some time to gel in the collective consciousness of society. As we look back in another ten or twenty years, we might be able to see the lines of the canon a little more clearly in this subject area.
I think Tim O'Brien's short story will come to be seen as a "classic" in time. The determination will be made if/when future readers will be to make connections and gain insight between the Vietnam War context of the story and their own worlds. Certainly in terms of style and literary merit, the work is virtuosic.
While I haven't read the book--though I'm now quite intrigued and have added it to my "to read" list--I will make the case for a changing literary canon. For years the "classics" remained fairly stagnant, with minor breakthroughs (eruptions, to some) in the field of women's and ethnic literature. I love and appreciate the classics, but there is room for other worthy literature. This sounds like it not only may qualify in theme but also in literary style. Being part of the so-called "canon" does not ensure a book will be taught or read, nor does its exclusion mean it's not a significant literary work.
I would agree - the way that it captures and comments on a precise moment in American history and the kind of comments and themes that it establishes make it a key work for study and a great contender for inclusion in the American literary canon. Literature often comments upon, reflects upon and commemorates specific moments of a nation's history, which this collection of stories undoubtedly does.
I would say its style does. It is one of the unique books that reads like a lengthy list half of the time as it tells a riveting story. The things they carried, both metaphorically of their personal burdens, and of the weight of the equipment each man must carry explores the relation between people and war and how they best cope with the circumstances they find themselves in.