I read something last week which discussed the idea that literature demands more work from its readers than popular fiction. And by "work" this piece really seemed to mean "work". And I'm going to pick on the semantics here. Because I think their whole attitude is wrong.
Speaking in generalities, heavy literature usually does have more depth and complexity than popular fiction. But readers want a satisfying experience from a book. A reader who loves David Foster Wallace doesn't think reading Infinite Jest was work--they liked it because Wallace gave them what they were looking for in a book. In this case, as one customer put it to me, something nice and chewy. The same reader wouldn't like the latest James Patterson because Patterson isn't giving them that satisfaction–they're missing their "chewy."
From the other side, someone who loves Patterson isn't reading Wallace because Wallace is too much work--they're not reading him because they think he's boring. To find a book satisfying they need something quite different from the Wallace reader –they need suspense, a plot that compels them to find out what happened next.
Because of the layers in most serious literature, and because it is largely read by intellectuals, the popular trope is that reading it is using more of your brain than reading popular fiction does. Well, it's certainly using different parts. But when I think of serious literature, I usually think of books that are commenting on the human condition as we find ourselves today (or whenever it was written). But I would argue that reading popular fiction uses more of your creativity. After all, the big draw of popular fiction is that it is escapist--it takes you to a place that is totally outside your ken. And imagining that, learning the different rules that exist in that particular world, is a more creative exercise for the reader.