The Magicians starts with a blatant reference to C.S. Lewis' Narnia series: our somewhat neurotic hero Quentin Coldwater (Jason Ralph) is a huge fan of a book series called Fillory and Further where three people (in their teens) find their way to another world - called "Fillory" - by stepping into a grandfather clock. I haven't read the books this is based on, but the author Lev Grossman has acknowledged The Lion, the Witch and the Wardrobe as a huge influence; so it would seem this is both intentional and originating with the author (rather than the TV writers). By the end of the first episode, the references to Harry Potter have also been made abundantly clear, as Quentin is singled out by "The Beast," the evil dude who already knows the name of our magical freshman. But he's already got a sidekick, an attractive young woman who knows a lot more about magic than he does: her name in this series is Alice (Olivia Taylor Dudley), but the name "Hermione" might understandably slip into your mind. But to make sure we know it's NOT Harry Potter, two people have sex (floating in the air) by 30 minutes into the first episode.
And thus we have the reviews that say "Harry Potter for adults." What they don't say is "Really - this is Harry Potter, just with some adult elements. And better written."
The falling-out between Quentin and Julia (played by Stella Maeve, Julia had been Quentin's best friend since childhood) in episode 3 was a huge problem for me: the series started so well, and that simply didn't fit. I get that both of them are messed up, but they'd been inseparable friends since childhood and to both lay into each other like that didn't seem likely to me. As we progress it becomes clear that needed to happen for future events, but it didn't really sell me on that when so much of the rest of it has such better human interactions ... I'm also really not a fan of Stella Maeve's acting.
Another Harry Potter similarity: the attitude about magic is "mess with this shit and you die ..." which I have a lot less problem with at the University level, so Brakebills University makes more sense to me than Hogwarts high-school equivalent. Harry Potter expects adult-level responsibility out of children, which never made sense.
But then there's the ending: after 13 episodes of roughly 50 minutes each, they don't wrap anything up. In fact, they leave everyone's lives far more miserable, with most of our main characters mangled and dying. I have a MAJOR issue with cliffhangers. This was also the terminal deal with Julia's character: what she does in this episode caps the least believable story arc of any of our main characters with another choice I failed to believe, leaving me, in combination with the cliffhanger, with zero interest in going on with the series.