What is your story about?

In a couple of his how-to books, Orson Scott Card gave one of the best bits of writing advice I’ve ever come across: Figure out what your story is about, and shape the ending accordingly.
Specifically, he says it can be about one of four things:
•Setting. Exploring world your characters live in.
•Question: Who murdered Roger Ackroyd? Who threw the overalls in Mrs. Murphy’s chowder? Why is the sun dying?
•Character: Character faces a dilemma and tries to change, or not change.
•Event: Something happens to the protagonist (s). Protagonists respond until they resolve the problem.
(I expanded on this for an article I wrote for The Writer several years ago, adding what I think are other “abouts” such as theme, or humor, but I’ll stick with Card’s basic foursome for now).
Very few stories are just one of these four things, but one of the four predominants. On TV, Perry Mason was a mystery (question); LA Law was a setting (it’s about the world of the courts, and of life at a high-powered law firm, not any individual character or case); Eli Stone was, I’d say, character, as the protagonist changes in response to his visions; and The Firm was an event story.
The point Card makes is that if you start with a character approach—a widow tries to keep going after her husband’s murder—that’s the arc the story has to resolve; if you end with the killer caught but the woman’s emotional story unresolved, you’ve failed. Conversely, a straight mystery (a question story) which gives the widow a new love and forgets to identify whodunnit is likewise going to disappoint readers. A story where the detective gets shot out of the blue will annoy almost everyone, unless it’s a Hill Street-style police procedural, where the point is that in the police world, people do get shot randomly.
Whenever I finish the first or second draft of a story, I sit down and try to figure out which of the four (or my added categories) it falls in. That helps me keep the story arc aimed toward the appropriate ending and not get swallowed by extraneous story elements.