I used to assign SF works to one of the two broad categories, loosely labeled "fun reading" and "food for thought".
"Fun reading", for me, includes many of the space operas, a.k.a SF fairy tales: quick page-turners, full of awesome glitter and breath-taking adventures, but once you are beyond the last page and yet another impossible world begins to fade in your imagination, the details of the story get quickly forgotten.
"Food for thought" might be quite a hard reading, not done overnight. But these stories tend to stick around, often for years. They tackle issues which don't get easily solved, or for which the solutions appear to be quite unsatisfactory, and / or these issues look plausible enough to make a permanent dint in the reader's mind. One well-known example: dystopias. One more is when the depicted worlds are only slightly different from the world we are used to, but the nature of the difference touches the very fundament which we have always assumed intact.
The Left Hand of Darkness by Ursula K. Le Guin, to me, is one of the rare books that manages to combine an easy-to-follow narrative with a deep story behind it.
It rethinks one of the most basic phenomena in the human civilization: the gender, by showing a world where "static" gender, in our definition, does not exist: its inhabitants periodically become either male or female, depending on the subtle circumstances normally beyond their control. For the most part of their lives though, they are neutral and neither engaged nor interested in one of the most profound drives of our own civilization: sex.
Other than that, Gethen, the world described by Le Guin, is not at all dissimilar from ours. It has a traditional empire-like state and a more modern ("efficient", as they say) totalitarian regime. There are several religions, which aren't completely unlike the ones we have. They have creation myths and the legends about love and vendetta. People could love or hate each other, get rich, get poor, take revenge, be an adventurer or a home-sitter. Everything is the same, only the gender-related issues - family, attitude to relationships, child-rearing - are quite different. No gender dualism, no weaker or stronger sex, no sticker to put on a newly born child. Also, the Ice Age on Gethen hasn't ended yet, and the inhabitants had to deal with it for millennia.
The book follows a story of the first envoy of the interplanetary society, the Ekumen, who tries to include this strange world into the broader community of other human worlds. Half of the narrative is from his perspective, another half is from the perspective of the local person that happened to be closely involved, so there is a discovery process going on from two sides.
The premise is that all the human worlds were once created by some proto-demiurg civilization, and that civilization also took the liberty to tamper with human genetics in a number of different ways. Gethen, though, is unique in their gender curiosity. From their own perspective, the inhabitants of all the other worlds seem to be perverts: forever stuck in one gender, like animals on their own planet, by the way.
(Which also made me wonder, what if the tables were turned? What if it was the human society which was unique in their "gender rigidness" as an experiment? Did Le Guin considered such possibility?)
I am not going to retell the whole story here, it deserves to be read and re-read. What amazes me is that the book, even though it was written quite some time ago, doesn't feel outdated (which is an achievement in SF land). Also, the level of detail is quite high. And most importantly, the characters behave in such a way that it's possible to empathize with them. Strange as it is, that world seems to work. And then it's easy to ask oneself, what would be changed in our world under such conditions? How much of our history, philosophy and whole culture would've been changed beyond recognition? How should people feel when they are oscillating between two extremities at a whim of a chance?
I do wonder if anyone would soon dare to make a movie out of it. Showing the whole society of not-quite-men-or-women without making it either a caricature or an abstract concept is a big challenge. But hey, isn't it what the SF is (or should be) about: imagining something that is improbable, but not impossible?