In a Smithsonian magazine article, Yale Professor of Law Amy Chua says that one of the many reasons behind the rise of what she refers to as historical "hyperpowers" - the Persian, Mongol and Roman empires, for example - is their tolerant stance toward a wide variety of people. What she means by that is that regardless of ethnicity, those empires offered individuals the opportunity to participate, to contribute and to rise in those societies. As she explains in the interview, marginalizing groups of people is an enormous waste of resources.
Elsewhere, in this YouTube she recounts how, having just completed an earlier book, World on Fire, she heard then French Foreign Minister Hubert Védrine refer to the United States as a "hyperpower," which raised questions for her about how hyperpowers come to dominance and how they ultimately lose their influence.
I like how this review of Day of Empire frames the issue:
[I]n the 21st century, 'empire' and 'superpower' and 'hyperpower' are terms that may require rethinking. They suggest boundaries, borders — even as they connote the expansion of territory and influence. But most of the powerful forces, good and evil, of our new century are borderless, globalized — the almost unimpeded global flow of information (images, ideas, news, music, movies, emotions, hatreds), products, commodities, capital, environmental pollution, climate change and terrorism. Perhaps, eventually, nuclear terrorism. In such a world, an idea (a rage, a grievance, a difference of cultural perspective) may create a superpower without borders, using a cave in Afghanistan or Pakistan as its Pentagon.
For a look at power from another angle, check out Paul Collier's book "The Bottom Billion," which describes what factors keep some countries nearly powerless. Ethan Zuckerman offers a thoughtful review here.