I would compare this book to a soggy awning. 
 You know when it rains a lot and there's that sagging mass where all the water has collected in the centre of the awning? I think that's a fair comparison to this book, in at least a few ways. 
 I should probably mention I wasn't partial to this book before I started it, but being the literary masochist I am, I put myself through it just to give it a fair judgement; I did do exactly the same with the entirety of the Twilight Saga, though I must say that this is preferable by far. 
 I compare it to a rain-drenched overhead covering for the following reasons: 
 - The plot revolves around a book called An Imperial Affliction. Said book does not exist. Even if we get a roundabout overview of the plot, it doesn't stop it from seeming like the basis of this book is made of Victoria sponge or a bouncy castle, rather than of concrete. Of course, this may well be subjective. 
 - Another subjective annoyance is the pop culture references. To me, this kind of eliminates a book from being eligible for "classic" status. Feel free to wholly disagree with me on this one, but I fear for people who aren't confident readers, and who become isolated from a book because they don't understand the references - what happens in fifty years if Natalie Portman and V for Vendetta have become far-distant memories? 
 - At first, I was drawn in by John Green's signature casual prose, which was a large part of my enjoyment of (as of now) the only other novel of his I've read, Looking for Alaska. On the outside of the awning, the stable, drier part and the beginning of the novel, it starts off with the same warm writing style that really is a pleasure to read, and is definitely a trademark of Green's books. But as I continued into the latter half of the story, it just sort of faded for me. I stopped caring - and it doesn't help I found most characters blander than mattress foam on dry toast. As many people have said before, the characters are defined by their illnesses (which makes it a little ironic that Green lampshades and tries in vain to subvert this). 
 And yes, it is indeed pretentious as hell. Green lampshades this at various points, but it doesn't stop it from being suffocatingly unrealistic. Perhaps it would have been "okay" if it was limited to only one character, but when both Hazel and Augustus and Isaac start spouting (sometimes clunky) dramatic prose, it really takes you out of the action, especially if it feels like it was thrown in for the sake of it. 
 Minor spoilers here: (view spoiler)[ I found Hazel's harsh judgement of the other mourners posting on Augustus' Ambiguous-Social-Network Wall rather distracting. It isn't particularly fair to people if they're genuinely upset about someone's death, but are denounced if they don't spend time with a person in their last few weeks. What if they didn't know? What if they weren't comfortable with just showing up in someone's life as soon as they hear they've got cancer? What if they live in Whitehorse, Yukon Province and have no travel funds? If I found out a friend of mine from primary school was that ill, I'd be devastated. It doesn't mean it would be appropriate to jump to their bedside after seven years of no contact. (hide spoiler)] 
 So yeah. In short, don't believe the hype. 
 Just a minor thought: parents in YA novels never get much characterisation; they all seem to be the same cardboard cut-outs as each other, maybe with a weird character quirk here or there. 
 Stop Bland Parents In YA Novels 2016.