This book firmly belongs in the camp of "Soft Science Fiction." What technology and science existed in the novel were there merely for background. Instead, she explores the meaning (or our preconceptions) about gender. This is accomplished through a species of Man inhabiting a forgotten colony world that has no gender. Or rather, has both genders in one. It would be a mistake to call these people hermaphrodites, since their sexual role (as either male or female) only manifests itself in the short period of "heat (called "Kemmer)," with either individual capable of going in one direction or another.
Into this is thrust Genly Ai, an emmisary from the Ekumen, a cultural and trade alliance uniting the lost colonies of mankind after some sort of unspecified disaster (war?). And of course his struggles to deal with these people who could be both men or woman -- often at the same time.
I must admit wrapping my head around such a gender disparaty was a challenge. Perhaps it is my own ideas about gender roles that intruded itself on the narrative. I found it difficult at times to remember that the characters (other than the protagonist, which is male). It was easy to fall into assigning those gender roles, since most of the characters are either gender neutral (and thus because of my male oriented bias I naturally assigned the male gender to those characters), or show a slight male bias. It didn't really start falling together for me until Genly and Estraven began fleeing across the ice, and Ursula challenged the reader with the two characters closeness.
Nonetheless, I also felt the book was too short, and there were issues I would have liked to seen explored more. I'm not convinced of Ursula's handling of (for lack of a better term) mating customs. There is no marriage per se, and when individuals feel the "need" they seek out others in Kemmer and ... well... let nature take its course. Families, therefore, are clannish rather than nuclear, with participation in rearing by other clan members (except in Ogoreyn, where socialism seems to have taken root, and children are wards of the state or such). I'm not so sure such types of organization would necessarily work, especially since my biological need to protect and nurture my daughter (as a father) is a very powerful one, and I can't imagine a society in which all fathers would not feel a similar attachment (despite the fact that there are plenty of fathers today who apparently feel no special attachment to their own brood...). I think a bit more exploration of this aspect was necessary.
Another aspect is that Ursula introduced a sort of supernatural mental abilities into the book that didn't serve any purpose except to show that despite the mutual affection Genly and Estraven had, such a level of intimacy (I think here Le Guin may have been making a point about the "mentalities" that separate men and women) is still not possible or sometimes even desirable (but at times -- especially when under stress -- become natural). The Gethenites (as the natives referred to their world) developed independently their own traditions of super-ordinary mental abilities (mostly focused on prescience it seems). The questions I would have is why did they develop abilities in this area, but not in the area of mindtalk that Genly's traditions developed? How does this relate to their unique gender? Is fortune-telling a feminine trait (doubt it)?
In the end, I think it would be illuminating to talk to a woman that had also read this book independently, and see if her perspective is different than mine, and to explore the book more fully.