There are thousands of elements that make each great story "great." I'm sure every editor out there could name the important must-have element to a great story, and not a single one would agree. For some it's fabulous settings, for others its intricate plots, relatable characters, humor, pacing, emotional arcs—the list could go on endlessly. Which is why you might be surprised when I say I've found the definitive element: interruption.
Okay, yes, you need to have some interesting characters, or at least it helps. But really, what you need to have is at least one person (and I'm sure some great authors out there could get away with making the "person" a setting and get away with it). That person needs to be doing something. That activity needs to be interrupted by something. Just when they get going again, another interruption, and on and on. It helps in the interruptions are interesting. It makes it even better if those interruptions escalate against the person's primary goals, raising the stakes so that each interruption causes more turmoil. But the main key is interruption.
I don't necessarily mean that an interruption always builds conflict, either. Rather, I've found that the authors I enjoy reading the most (Katie MacAlister, Janet Evanovich, Laurell K. Hamilton, Kim Harrison, Charlaine Harris) never allow their characters to get comfortable. They never allow a scene to unfold as it appears like it's going to from the beginning. The conversation gets interrupted by action, the sex scene is interrupted by conversation, magic interrupts a routine journey, an uninvited guest interrupts a quiet evening at home.
Of course, the interruptions happen during bigger scenes, too. But the point is nigh constant interruption. The more interruptions, the more story you get. Take Harrison or Hamilton as examples of this. Five hundred to seven hundred pages to each of their novels, yet the main characters live through an average of two to seven days maximum in each book. Two to seven days of my life, with its easy routines and lack of virtually any interruption whatsoever, could be fit quite easily into a few well-constructed paragraphs.
More interruptions equals more story. More story equals happier readers. Happier readers equals more book sales. More book sales equals more popularity. More popularity equals more money.
In other words, if ever you're at an impasse in your story, interrupt the main character. It worked fabulously for Douglas Adams, it works for Evanovich, it works for J.K. Rowling. It'll work for you. I'm definitely going to make it work for me.