Literary agent Donald Maass (who represents several prominent authors) discusses this topic in "Writing the Breakout Novel". It's an excellent read, and I hesitate to attempt to summarize a rather lengthy book, but a couple of the elements he mentioned are:
Larger-than-life characters
Heightened stakes
Unexpected moments of grace
Beautiful language
I'm not sure whether I came away from the book able to write better novels, but I certainly came away from it thinking that Donald Maass is a very, very smart man.
