The theme of magic actually being real is not an original or new one. It has been done several times before, most prominently by JK Rowling's Harry Potter series.
What Grossman brings in his book is perhaps a more realistic, less idealized presentation. His book is filled with characters that perhaps reminds one of people we knew in school, or have heard of. While he doesn't go into extensive detail about how magic works, the book emulates a basis of knowledge that suggests that, while magic itself is mysterious, the ability to manipulate it is known and formulaic.
The story is divided into to main elements: the tutelage of the main character, Quentin, at a sort of "University of Magic" called Brakebills, and the second part details their discovery that the "magical world of Fillory" (a sort of CS Lewis analogue) is in fact real, and they are able to travel to it.
It's always fun finding the theme or intent of the author in writing his story. For some writers, there really isn't one -- the story is for pure entertainment. Given Grossman's status as a literary critic (so sayeth the jacket sleeve), I can't imagine the book would not have that level of depth.
One of the elements we discover about Grossman's magical society is that it is essentially one that takes care of itself. Once the students graduate from Brakebills, all of their expenses are paid (secret magical manipulation of the world economy), and it is ultimately up to them to discover their own way in the world -- there is no "job placement program."
Once Quentin graduates from Brakebills, he rapidly descends into a decadent existence, seeking pleasure for the sheer purpose of experiencing pleasure. Daily him and his companions drink, or engage in drug use, throwing parties just for the sake of having one, or otherwise engaging in activities for the sole purpose of filling in the time in their purpose-less lives.
We see a precursor of this when Quentin visits his girlfriend Alice's parents. They engage in seemingly pointless activities, such as remodeling the house based on Roman era architecture, or studying the musical themes of pixies. Both of Alice's parents seem to be disengaged from the world, wrapped up in their own make-busy work. Alice makes Quentin swear that they will not end up like that. But perhaps that is unavoidable...
When the gang discovers the ability to travel to Fillory, they find that it is a much darker and more dangerous place then they had imagined. It kills one of the party, and crippled Quentin. What should have been Quentin's greatest dream is ultimately poisoned by the issues the humans bring with them.
I think ultimately Grossman's theme in this book is that we must create a purpose for our lives by ourselves. Despite the magic, despite Fillory, and despite the Eloi like existence, Quentin is ultimately unfulfilled because all of these elements are tools for him to use and create a fulfilling life. This is contrasted by his girlfriend Alice -- stronger in magic than any of the other characters -- who uses magic as a tool and not an end, and gives her life purpose at the very end.
I can't disagree with this viewpoint at all. Having all but rejected religion, and the idea that the purpose of my life must be defined by some external power, the idea that we as individuals give our own lives meaning -- create it based on our own interests, ideals, and experiences -- is far more appealing than having it dictated on-high. I am sure there will be those that disagree with this viewpoint, but of course it is my viewpoint.