(Disclaimer: no, this is not a post about politics, or political correctness...;-)
This is an attempt to pinpoint one of the things that makes a story resonate: that is, one of those qualities that makes a story stay with the reader long after she/he has finished reading it. I'm aiming at what should be left unsaid in a story.
(Of course, there are more aspects that can make a story unforgettable: compelling characters, intriguing setting, breathtaking prose, perceptive plot, mind-expanding concepts, an unrelenting tension arc, an intense ethic dilemma, razor-sharp imagery, and more . Of course, I won't pretend that this list is complete. Furthermore, what works for a particular reader also depends on personal taste.)
There is a certain balance in what a story should tell, and should not tell. It is related to the question if a story should wrap up each and every bit it puts forward, or leave an open ending.
Side note: admittedly, some readers prefer a well-rounded off story. However, I would argue, such a story, unless it contains one or more of the other aspects -- mentioned above -- in abundance, will not be a highly memorable story.
To make matters more complicated (and keep in mind that the matter is complicated, as there is no 'how-to-write-the-perfect-story-in-three-easy-steps' guide, although the sellers of such guides want you to believe there is, but before you're tempted to actually buy such a guide, I'd strongly recommend you read this), the amount of ambiguity required depends on a variety of factors, such as -- but not limited to -- the subject matter, the theme, the thrust of the narrative*, and the story's sensibility and/or ambition.
(*: I almost put the writer's intent in there, as well, but if a writer's intent is to create ripping yarn rather than a thought-provoking dilemma, to write more for entertainment's than for art's, provocation's, or even complexity's sake, then one might expect the story to be more forgettable, and be wrong because the story contains a colourful world, enthralling characters, or other aspects that will make it resonate.)
Roughly speaking, on one end of the spectrum there is the story that does not stand alone: an obvious novel excerpt, a part of a larger narrative (or series) that falls flat without its larger reference frame. The ending, or story itself, is not satisfactory, because too many pieces of the puzzle are missing. I've rejected some very intriguing stories exactly for this reason, albeit reluctantly.
On the other end of the spectrum there is the perfectly rounded-off narrative: every question is answered, every thread is wrapped up (no loose ends), the dilemma is answered, and the ending is definite (be it happy or sad). I've rejected much more of such stories, and with a lot less reluctance, which should give you an indication of my preference.
There is a vast, and uncertain, middle ground ranging from 'raises more questions than it answers' to 'wraps up almost everything too pat'. With the addendum that there are always exceptions to the rule: sometimes an intentionally ambiguous story works because (or in spite) of it, and sometimes a perfectly encapsulated one rings so true that it stays with you.
So the writer must find a balance: she/he should intrigue the reader, but not leave the same reader stranded in a hall of smoke and mirrors (unless your name is Chris Priest...;-). The writer should round the story off in a matter sufficient to give the reader a satisfying reading experience, but also in such a matter that the reader hungers for more.
And, to make matters worse, some readers hate to be left in the dark, while some other readers hate to be led by the hand every step of the way. So go figure.
Therefore, I think it is very important that a writer trusts the reader's intelligence and willingness to think along. Provide enough, hopefully subtle, hints to make the narrative work, but -- and this is the heart of the matter -- leave enough unsaid to make the reader work it out for her/himself. Exactly how to do that is one of the fine arts of writing.
"So," I hear some of you complain, "can't you be more specific?" Basically I can't, because each and every story is -- should be -- different.
Because I assume that a writer wants to be unique, with a distinct voice. So each of your stories should be unique, with a distinctive tone. So each story should have a different mix of elements that make it work. So the amount of what to leave unsaid will be different for each story. Most importantly, it should be there.
Thus, the fine art of leaving things out.
In general, a lot of genre stories tend to overexplain: world-build 'till you drop, infodump like a ton of bricks, caricature rather than characterise, and ram the message home. I'm not saying that there should be no world-building details, infodumps (sometimes these are inevitable, but that is much rarer than most of us think), characterisation (instead of cardboard puppets), or even a moral; but try to use only the telling details, the essential infodump, the vital yet conflicting character traits, and make the moral subtle and ambiguous.
There's also a tendency to tell a story from a first person point-of-view, so that the readers are privy to the main character's thoughts, and the writer is tempted to explain all the main character's motives this way. That's lazy, and is one of the reasons that first person PoV is hard to do well (of course, you can, such as in Andrew Humphrey's "Open the Box": a story that first repulsed me, but which I couldn't stop thinking about, and impressed the hell out of me on rereading. It leaves a lot unsaid, and forces you to consider the roots of racism, if you're prepared to go the distance), as it's often better to show a character's motivations through the character's actions rather than through the character's direct thoughts. Even then, don't show every tidbit, but only the choicy parts, and also show conflicting actions: people are rarely consistent in their actions, and make wrong decisions on the spur of the moment.
In essence, every good story is a collaboration between the writer and the reader: the writer creates a framework on which the reader can build her/his imagination. A story can't simulate reality (reality is too complex for that: nothing that is less complex than reality can perfectly simulate it. A computer simulation that would duplicate reality would be at least an order of magnitude more complex than reality itself), but can evoke it, trusting the reader to fill in the gaps. One of the paradoxes of writing is the creation of nothing, that is: leave things out. In that vein, doing nothing just right is hard work (and it's not an oxymoron).
Make work avoidance work for you!
(Note: these thoughts came to the fore after I was reminiscing about a little discussion I had with Paul Raven, Sean Green and Gareth L. Powell on Paul's Velcro City Tourist Board about a drabble competition Paul held with a copy of Glorifying Terrorism as the prize.)