I could answer your question by saying that "American" is a characteristic of poetry written by Americans, but that would only generate further questions: What is an American? What is poetry? What is really American about American poetry is that it so often generates questions like these, continually interrogating itself, continually attempting self-definition as an entity distinct from other poetries.
Despite the certain abundance and apparent variety of American poetry, it may be that there really are only two kinds of American poets, distinguishable from one another by their contrasting attitudes toward history: one kind of American poet accepts its weight, the other writes to annul it. The tradition of American poetry consists of the need to find oneself on one side or other of that divide.
The choice (if it really is a choice) between an awareness of history and the desire to repeal it and begin over again determines in part the themes of American poets and the ways in which they write about them. That choice also affects a poet's relationship with the audience for American poetry, an audience most often characterized, these days, by its problematic nature; either it is absent (having chosen to go shopping at the mall) or present (in the classroom) under duress, but for credit.
The most serious issue facing American poetry today is the relationship between poet and audience. To take Ezra Pound's side in his quarrel with Harriet Monroe over the quote from Walt Whitman that adorned the cover of Poetry magazine in its early days, great audiences are not necessary to the creation of great poetry, but some audience is certainly necessary, and the larger and more knowledgeable the audience is, the better for any art, including the art of poetry. In our time, the creation of such an audience for American poetry has to become the responsibility of poets.
The tradition that I write out of is the American tradition as I have described it above, and I think of myself as someone who chooses to write a poetry that attempts to sustain the weight of history, to bear the past into the present. That tradition therefore includes as much of the past as I am capable of carrying forward, however little it often seems to be.