Toni Morrison famously said that if the book you really want to read hasn't been written, then you'll have to write it yourself.
No, it's not as easy as she makes it sound, of course. But I think there's a lot of truth to it. Many times I've experienced a faint disappointment with an otherwise entertaining story - a feeling that I might have preferred a different ending, or another middle, or else I would have changed the fate of the antagonist, or the protagonist's past, or another character's principal weakness, or what-have-you, in order to make the story more satisfying. I'm pretty sure we all feel like this sometimes, even though we might thoroughly enjoy the story while reading it (or watching it as a movie).
And I think this is the difference between the story that is entertaining, but ultimately forgettable, as opposed to the story that stays with you forever.
I do realize this is a matter of taste, but to me, some of the elements of an unforgettable story are: a protagonist that is likable and good, a hint of mystery, a plot that reveals layers and secrets and surprises, danger of the world-changing kind, but also friends and safe havens (fantsay fans, think of the safety and cameraderie of Elrond's house, or the cosiness of Gryffindor House) and then, a happy ending with justice served and no loose ends. And even with all of these things present, there has to be a certain story magic that brings it all together. That is the story I want to read.
If only I had that story to read today! Or, as Toni Morrison says, if only I would just write it.