I think that Zakaria would point out that one of America's universal strengths is the ability to read and assess situations better than most nations.  The spirit of pragmatism and practicality that has been almost a part of American identity in both politics as well as in its history has been a source of optimism, and is something that Zakaria sees as a continuing source in the new "post- American" setting.  The idea of America altering its approach to working with world powers in a globalized setting is a critical element in Zakaria's optimism.  America must adapt its own approaches to dealing with the world, recognizing that actions cannot be taken separate of how they impact other nations around the world.  The days of unilateral action in the name of exceptionalism might be gone, and the recognitiong of this could be one source of optimism in Zakaria's work.  When he speaks of moving away from "hard power" demonstrations, it is in this idea that American interests and political acknowledgement of other nations is done in a manner that does not seek to isolate or dominate other nations in the world, but rather use the globalized and interconnected nature of the world to work with these nations. 

I would also say that Zakaria sees optimism in American innovation.  Both in the book and in his own thinking, Zakaria believes that American education and, in particular, higher education is the source of American greatness and optimism.  He believes, and is right to a great extent, that American colleges and universities are the source of ideas that are transformative.  At the same time, while "the rise of the rest" is evident, many students who seek higher education opportunities will still seek to study in the United States.  While the world demographics are changing, Zakaria believes that American education will still be one of the defining elements that will make the United States indispensable to the progress of the world and its own stature in this pursuit.