Greene took an almost ungraspable concept and delivered it to the lay person with relative panache. Giving practical, macroscopic examples of relativity and spacetime; breaking down the concepts within quantum theory; taking the various forms of string theory and mentioning the compilation of those theories to M-theory, makes Greene gifted.
Unfortunately, there are digressions and obvious gaps that most readers are trying to come to terms with, which Greene doesn't even mention. Oftentimes, many people reading books about the fundamental nature of matter within our universe, search for an answer to the question of why matter manifested the way it did? This question is left completely unanswered. It took an engaging conversation with a close friend to realize that that question is simply impossible to answer and therefore completely irrelevant. It would have been nice for Greene to have stated that, however.
Another "elephant in the room": while these concepts are truly phenomenal, they're also entirely theoretical. It's astounding that extraordinarily complex multivariable differential transforms, which require months for computers to process, based upon algorithms designed to answer our questions, can result in simple integer solutions. But, can we reliably say that these computer simulations give us a numerical answer that is truly representative of the physical universe? No. We can only hope it does. Therefore, it begs the question, while we search for the elegant unifying theory of the universe, when will we realize that we either come to accept elegance is lost in the complexity of our [fabled] search for unification, or finally realize we're approaching the idea all wrong?