What are the characteristics of postmodernist literature that makes it challenging to read?
Irony has lost both its humor and its drive to correct social wrongs in Postmodernist literature, so the ironic tone that might amuse in Austen's Pride and Prejudice becomes a biting, humorless sarcasm in postmodern works. The Fault in Our Stars is a good example of this postmodernist stripping of humor and intent from irony. This is one characteristic that makes these works challenging to read.
mostly friendless, ... exploiting his cancertastic past, slowly working his way toward a master's degree that will not improve his career prospects, waiting ... for the sword of Damocles to give relief that he escaped lo those many years ago.... (John Green, The Fault in Our Stars)
Even more challenging is what postmodernist Marxist critic Fredric Jameson calls chronological, or temporal, "schizophrenia." Until modernists and postmodernists, chronology in literature was fairly straightforward following a cause-effect order through time. Flashbacks were employed to tell backstories or to fill in events that transpired away from the point-of-view focus, but temporal cohesion was kept intact. An example of such a flashback occurs in Austen's Sense and Sensibility when Willoughby bursts in on Eleanor and tells her the story of his away-from-point-of-view events: he is speaking about a time he flashes back to, so temporal consistency is not lost, only momentarily redirected.
"I do not know ... how YOU may have accounted for my behaviour to your sister ... [but] When I first became intimate in your family, I had no other intention, no other view in the acquaintance than to pass my time pleasantly while I was obliged to remain in Devonshire,..." (Jane Austen, Sense and Sensibility)
Modernists, like Virginia Wolf, chose to show fragmented time, showing things that are happening simultaneously--like Mrs. Dalloway's actions simultaneously occurring during Septimus's experiences in the park--by using real-world commonalities, like an aeroplane they both see performing the same act. Postmodernists fragment time even more severely because they don't have the modernist's regret at the post-World War fragmentation of spatial-temporal (space-time) experience. Postmodernists disrupt timelines with regularity, with simultaneity, flashbacks and flash-forwards, creating fragmented, discontinuous timelines.
Space doesn't permit a full discussion of Postmodernism's challenging characteristics, but two other prominent ones are authorial intrusion and self-reflexivity. In postmodern literature, the author may choose to intrude into the fictive world and address the reader or another character thus creating a sometimes disconcerting partnership between the otherwise invisible author and reader. This intrusiveness may be in reaction to Barthes declaration that the author, as such, had died for purposes of textual analysis and all that embodies meaning is the text and the text alone. Similar to intrusion, the text may call attention to itself as a text by incorporating self-reflexive remarks about meaning, structure, or characters. An example of very boldly made self-reflexive remarks is in London Fields:
Three days in and I am ready to begin. ... Hear my knuckles crack. ... Hurry. I always assumed I'd start with the murderee, with her, with Nicole Six. But, no, that wouldn't feel quite right. (Martin Amis, London Fields)