By connecting the awarding of a peace prize with the concerns of a world in which terrorism has become a constant threat, Obama makes clear the exigency of his message when he says: "I do not bring with me today a definitive solution to the problems of war." Nobel laureates are given few formal constraints in terms of their responses but Obama faced the more general constraints of trying to meet very high expectations and the conflicting expectations of the peoples of different nations. I believe that he did a good -- though not perfect -- job in meeting these differing expectations, and so crafted a speech that served as a fitting response to the occasion.

Whether or not one believes that Obama achieved the Aristotelian concept of ethos -- the ability to make a credible ethical appeal -- depends probably more on one's own politics than the speech itself. I believe that he did achieve ethos in that he continually described the complexity of reality. One example of this is when he said: "… in many countries there is a deep ambivalence about military action today, no matter the cause. At times, this is joined by a reflexive suspicion of America, the world's...

This is appropriate because in the context of arguing the concept of a just war in a speech accepting a peace prize he is better served by logic than by passion.
I believe that for the most part Obama achieved Aristotle's standard of enthymeme in that his argument is logical and compelling -- in no small part (as noted above) because he recognizes that reality is complicated. He describes the fine line that nations must walk in such a mutable world as follows: "And even as we confront a vicious adversary that abides by no rules, I believe that the United States of America must remain a standard bearer in the conduct of war. That is what makes us different from those whom we fight. That is a source of our strength. That is why I prohibited torture. That is why I ordered the prison at Guantanamo Bay closed. And that is why I have reaffirmed America's commitment to abide by the Geneva Conventions."

I do not believe that it meets Aristotle's definition of a "commonplace" because it could not easily be used in any other situation.