"Difficult," when applied to literature, generally refers to works that are hard to read, hard to get through, hard to finish. And for me, that describes Left Behind far more than it describes Atonement—the second of which I read twice in quick succession and would read again happily, while the first made me hate life and hope to be transported mid-sentence to another, better realm, somewhere in the sky, beyond this world of sin and hardship and pulp Christian kitsch. There are certainly some highbrow books with sesquipedalian sentences that I've had trouble getting through—Melville's interminable, rambling collection of anecdotes-to-nowhere about the Galapagos Islands, for example. But is my desire to stomp upon "The Encantadas" until it can trouble me no more really categorically different than my desire to hurl John Grisham (or at least his books) from a height?
In the first case, I know, the fault is supposed to be in me, that I have failed to look deeply enough into the work of a master. In the second case, most people would say the fault is in John Grisham's shockingly vapid prose and brain-dead plots. But in both cases, the experience is one of repulsion, boredom, alienation. Melville and Grisham: They're both difficult for me to read.
So why should readers be open to labeling Fifty Shades as difficult (at least for some), rather than as just bad? There are a couple of reasons. First, I think it's more true to the experience of reading to see "difficult" as wrapped up in evaluation of "bad," rather than as separate. Most people are willing to admit, even if grudgingly, that aesthetic quality is to some degree subjective. You may (as I do) find Art Spiegelman's Maus a tedious, pompous slog, but that's a judgment about which reasonable people may differ (even if, of course, all right-thinking people agree with me.) But "difficulty" seems to hold out the possibility of more objective standards—to assure us that these books, over here, by Joyce and Faulkner, are 1000 pounds of pure prose, while these books over there, by Stephenie Meyer or Tom Clancy, are sniveling 90-pound weaklings of meretriciousness. It's as though "good" may be relative, but "tough" is always and everywhere the same.
Again, the problem with this is that it isn't true: "Difficulty," like "good" or "bad," is subjective. Some folks (okay, many folks) may be put off by Henry James's endless sentences and deliberately opaque social vacillation, but others may find it engrossing. Some people may love to flip their way through Jack London's manly adventures; others may find a novel's worth of cruelty to animals so upsetting as to be unreadable. Some people may love the sweep and romance of Gone With the Wind; for others, the unrelenting, vicious racism may be off-putting. For that matter, most reader surveys indicate that men, as a group, don't read fiction of any sort. Does that mean that men are serious-minded readers of non-fiction? Or does it mean that romance novels and mystery novels, those quintessential guilty pleasures, are, at least for some, difficult?