I first read The Left Hand of Darkness about four years ago, and I came away from the experience so very impressed. Since then I've listened to a handful of academic talks about one Le Guin novel or another, read a couple of student papers about her, and waded through a handful of scholarly articles. Conclusion? She's smarter than anyone who ever gets around to talking about her, with the possible exception of the redoubtable Fredric Jameson*, though rather more than the similarly redoubtable Harold Bloom.
Do I have more to say about LHD this time? Well, yeah, I guess so, but it's like what Johnson said of Gray, in a way: had she written always thus, it had been vain to blame and useless to praise [her]. Le Guin does a wonderful job of portraying the two quite different competing cultures on the planet Gethen (known as "Winter" to the representatives of the Ekumen, the coordinating body not unlike Star Trek's Federation), and the nuances of their different social spheres. As she remarks in her brilliant introduction to the novel, to her a work of science fiction is a thought-experiment that comments on the present. One of the nations is bureaucratic, petty, and randomly vindictive; the other is monarchic, chaotic, and enduringly vindictive. There's some clear social commentary on contemporary American politics and culture from the time (1969), especially on the intertwined questions of gender and sexuality.
Because you see, people on Gethen are neither male nor female. They are potentially both, and usually neither, except for a few days each month (a period known as "kemmer") when a person becomes either male or female. The same person can father children with another Gethenian, as well as become pregnant and carry children to term. They're humans, more or less, as the Ekumen believe that the previous Hainish civilization engineered the Gethenians and left them on Gethen to evolve in isolation, but their differing sexuality means that they've evolved different social structures, taboos, and intergroup practices. Le Guin's thought-experiment finds that without stable gender, for example, all-out war hasn't appeared, and marriage in the traditional sense hasn't evolved either, but the human incest taboo takes a surprising turn.
Environmentally, Gethen is at the border of human survivability, in its average temperature and climatic conditions. There are surprisingly few species of animals, too, since the Gethenians haven't evolved from the planet's original species, so there are some really fascinating passages (to my eyes, at least) about the outsider's absolute dependence on the insider's inherited knowledge. It's just such a unique, provocative book, that it pays back any time you want to spend on it.
*The Jameson link is to the original 1975 version of his article "World Reduction in Le Guin," which appears in expanded/clarified form in his 2005 essay collection Archaeologies of the Future, a book you should all read, except for Fraser, who would hate it. The full text of the 1975 special Le Guin issue of Science Fiction Studies appears to be online, too, so happy browsing there!