I recently finished reading Save the Cat, by Blake Snyder. (Loved it!)
Although it was chock full of useful info, the one piece of wisdom that really resonated with me was a single word: Whydunit.
That's the term the author uses for the mystery genre. When I read it, it was like one of those lightning bulb moments. Because, of course, duh! No one cares whodunit.
The most disappointing mysteries are the ones in which everyone has an obvious motive, and at the end of the book... well, it turns out one of them did it. For exactly the reason you thought they might. Boooo-rrrring!
Much more interesting are the ones in which the motive is obfuscated in some way, and comes clear just before the end. There are a ton of ways to do this... including the one I've chosen for my book, which I hope people will find interesting and surprising.
I wonder: are there other questions that can provide a really satisfying end to a mystery? Maybe "Howdunit," for a locked room mystery. Or "Howcatch'em," for a Columbo-style story, in which you know the guilty party from the start.
But I think it's the Why that really gets to the heart of mysteries... maybe to the heart of stories in general.