Over the weekend I finished Crime and Punishment. This book, when compared to something like Eragon, reveals what I feel to be the major difference between literature and popular fiction. It is not that literature is old, where popular fiction is new. I have read a number of old books that, although still read today, are more like popular fiction than literature; books like Around the World in 80 days or Treasure Island. It also has nothing to do with how widely read a book is. Harry Potter, among other popular works (possibly even Eragon), have been translated into multiple languages and widely read around the world. It is also not the presence of themes or insight. I am sure even smutty romance novels have themes even if they are shallow like: Evil people always die terrible deaths and people with true hearts find true love, or: The man you least expect will have large pecs and will sweep you off your feet.
We also make a mistake if we say the difference between popular fiction and literature is that literature is good and popular fiction is not. While tempting, this is just haughty. Popular fiction can be judged as "good" if it's popular, for that's the goal. Literature may or may not be popular in its own day, but works of literature are not judged on whether or not they make the New York Times best sellers list. I will not try to describe what makes good literature because I am just a student.
I think the main difference between literature and popular fiction is that literature is demanding while popular fiction is entertaining. A reader of popular fiction says to the book, "impress me, move me, trick me with that twist of plot." A book is considered enjoyable based on the experience of the reader. If the book made you laugh, cry, excited then it worked, it was good. The goal is to make the work as accessible and enjoyable to as many people as possible. Tools like subtlety are not just useless in popular fiction but rejected. If a reader misses something it is better left out. Everyone who reads a book of this type should have nearly the same experience, that is, they will understand or "get" everything. If any questions remain at the end of a book it is merely to whet the appetite for a sequel. The same is true of popular non-fiction. It must be understandable to the lowest common denominator or it will not sell. This alone should make any popular work that tries to teach you something suspect. Luckily, popular fiction rarely has anything to teach. If it did it might be too demanding.
In contrast, literature asks a lot from its reader. If nothing else it asks the reader to sit still and keep read. A lot of literature is not satisfying until the reader has finished. Authors like Dickens, Austen, and Dostoevsky have sections, sometimes long sections, that take discipline to get through. But the pain is worth the effort. Some things cannot be simplified, in many cases it is profane to try. In Crime and Punishment Dostoevsky deals with the role of suffering to expiate guilt, the torment of the soul and isolation caused by sin, how wicked people can do loving things (and how those loving things do nothing to ease the suffering of the wicked), and a host of other things I am sure I missed. Popular fiction must ignore these kinds of issues or simplify them beyond recognition. Like self-help books the best they can do is offer simple answers to complex and difficult questions. It is best if they leave such issues alone.
Literature is also demanding in that it forces the reader to think and reflect. If a "popular" work demands that a reader think about what is being said it is doomed before it hits the shelves. Thinking is work, and people who are looking for entertainment do not want to work while they read. The author should do all the work for them. In order to enjoy literature, on the other hand, the reader must engage the work. Literature is written to make people think. Sometimes it even asks the reader to change. That's a lot of unexpected work for the reader who approaches literature looking to be entertained. If the reader tries to pass over the elements in literature that demand reflection he will miss the most enjoyable parts, the very things that makes literature great.
To sum it up, Eragon can be read, enjoyed and forgotten. Crime and Punishment will not let the reader get off so easy.