In the years before things like Twilight and Hunger Games made it mainstream entertainment, about 80% of YA fiction was Lurlene McDaniel-style kids-with-cancer weepies. John Green revives the trope here. Because they are John Green characters, the kids with cancer talk like hip twenty and thirty somethings with self-aware wit and panache. (That's not necessarily a criticism.) The story of a budding romance between two teenage cancer survivors moves along predictably (again not necessarily a criticism) and the inevitable tears are plentiful and well earned. The wit displayed by all the characters throughout keeps the weeping from becoming overwhelming.
As YA fiction written for its target audience, it is perfect. For bookish, slightly off-beat teenagers, these characters are more clever, more witty, more culturally aware versions of themselves. And who that age doesn't like a good cry? As an adult reader, I recognize a certain sameness in all the characters and a lack of true drama, but it's not written for me. John Green knows exactly what he's doing and that's why he's one of the best working right now.