I think that the fundamental themes that comes out of the speech are what makes it so much of a "classic" text about what it means to be an American.  On one hand, I am not sure the discussion of race in American History can go very far without bringing the speech up and analyzing its implications.  It is here where I think that the speech occupies "Classic" distinction.  Dr. King provided the template for not just how he saw race relations in the time period, but also how the issue of race relations needs to be continually discussed in America of all time periods.  In a much more philosophical sense, the speech is a classic because it really probes into the complexities of racial identity in America.  How is the nation to understand both the acknowledgement of race and the transcendence of it?  The speech brings this out in such a powerful way that it almost compels the reader to adopt a higher and more elevated spiritual condition, which makes it a classical work.  Just like Jefferson and Lincoln, Dr. King is able to bring out a social and political condition and transform it into a moral and ethical one, enabling the contingent to become transcendent.  It is here where I think that the speech occupies such a central and classical position in American construction of identity.