At first, a large part of the novel seems to be an exploration of a neuter society—a society in which sexual difference plays no role, although love and jealousy remain. The world of Gethen has no history of war, but Le Guin has Genly Ai's narration state that the exact reason for this is unresolved. It could be due to the Gethenians' unique biology (the absence of a deep sense of duality implied by strong sex divisions may cause the Gethenians to lack a necessary component of nationalism), or it could simply be a side effect of the planet's harsh climate, limiting warfare to small skirmishes by simple economics.
Also related is the far slower pace of technological development. The Gethenians are mentioned as having gone through a very slow-paced and gradual industrialization, with many semi-feudal social institutions left intact, rather than the breakneck industrial revolution which Earth experienced since the 18th Century. In one episode, it is specifically mentioned that a particular type of truck has been in use for centuries, almost unchanged, and that Gethenians feel no special need to improve on it and develop a more advanced model. Like the above, this may be related to the absence of a sharp male/female dichotomy, or may simply be a side-effect of Gethen's meagre natural resources, which are deficient not only in pure raw materials, but also in certain forms of inspiration (Genly Ai speculates at one point that the Gethenians' failure to invent the airplane may be due to the planet's lack of any airborne animals).
The book features two major religions: the Handdara, an informal system reminiscent of Taoism and Buddhism, and the Yomeshta or Meshe's cult, a close-to-monotheistic religion based on the idea of absolute knowledge of the entirety of time attained in one visionary instant by Meshe, who was originally a Foreteller of the Handdara, when attempting to answer the question: 'What is the meaning of life?'. The Handdara is the more ancient, and dominant in Karhide, while Yomesh is the official religion in Orgoreyn. The differences between them underlie political distinctions between the countries and cultural distinctions between their inhabitants. Estraven is revealed to be an adept of the Handdara.
Genly is on Gethen as an envoy from the Ekumen. The envoy has trouble communicating with the Gethenians, because he doesn't understand shifgrethor. Using his ansible, Genly can communicate in real time with the Ekumen. In an attempt to prove that he is, indeed, an envoy from other civilizations, King Argaven asks off-planet correspondents of the Ekumen what makes a person a traitor. Although the message is received and answered, Argaven is not satisfied with the answer.
During Foretelling, a ritual of answering questions about the future, the Foretellers communicate in a deep and mystical way. Genly tries, with Faxe, the Weaver of the Foretellers, and later with Estraven, to engage in mindspeak, in part because he misses doing so, and in part because it is not possible to lie in mindspeech. When Estraven finally hears Genly's mindspeak for the first time, he is shocked and puzzled that Genly's mindspeaking voice is the voice of his dead sibling, because there are no lies in mindspeaking.