Isn't Grossman just ripping off JK Rowling and CS Lewis?
Not really. It's more of an homage. Grossman, who is also the book critic at Time magazine, wrote The Magicians as an intentional response to the stories he grew up with. As he said in an interview with Salon: "In a weird way I really felt that I was talking to J.K. Rowling and C.S. Lewis and trying to tell them about how my life was different from the lives of their characters. I had to explain to C.S. Lewis how poorly I'd been prepared for some of the challenges of early middle age by my obsessive childhood rereading of The Chronicles of Narnia."
So it's intentional on Grossman's part that Quentin and his peers grew up with the same magical stories and fantastical frames of reference that we did ("Avada Kedavra," one character jokes wryly in the second episode), and his books are both a knowing pastiche of fantasy tropes and a way of turning them on their head. While fantasy narratives promise escape, The Magicians highlights the danger of that escapism. Unlike the fantastical realm of Harry Potter, where Harry is the chosen one who uses his powers to save the day, The Magicians suggests that maybe there's no such thing as a chosen destiny, that nothing in life comes easy, and that there's a real danger in looking for fulfillment in worlds beyond our own.
So that's what you meant by "adult" themes?
Yup. Not to give away too much, but there are some very dark ideas at work here. Quentin begins the show in a mental institution (a tweak from the book, but one that packs a punch), and much of the show is a metaphor for -- dare I say it -- millennial alienation, and what happens when you realize that getting everything you wanted as a child isn't everything after all (or, on the flip-side, when the sense of possibility and creative potential you felt as a child begin to become closed off to you). Julia's story in particular is a powerful allegory for professional disappointment in the adult world, as this type-A young woman who was told she could do anything experiences the equivalent of being rejected from the Ivy League (later, her study of magic turns out to have a lot of parallels with drug addiction). Meanwhile, at Brakebills, the going is just as tough. Magic, it turns out, isn't about simply flicking a wand -- it's tedious, repetitive, mind-numbing work, sometimes with brutal consequences. As Quentin's friend Eliot (Hale Appleman) is fond of reminding him, "magic comes from pain."