I think that one of the most basic elements that comes out of Whitman's Preface is the idea of linking the subjective with the national.  Whitman seeks to establish that there is such a thing as "American character" and this can be brought out through poetry and art.  Yet, he does not assert this notion as one singular notion.  Rather, he embraces the spirit of the poet and his own sense of artistic freedom in being able to articulate different narratives and "umbrella" all of this under the idea of what it means to be in America.  As "the United States is the greatest poem," Whitman seeks to establish a domain where different and divergent experiences are fused into one notion in terms of what it means to be American.  If this is valid, then I think that American Literature has done this and continues to do so.  We fully embrace the idea that being "American" carries with it different conceptions of the good.  This plurality to American identity allows us to envision America in the broadest of ways with a foot in both personal expression and national character, something that comes out of Whitman's thinking.