It would be impossible - or at least make for an excessively long review - to enumerate the ways in which in this book is incredible. This is science fiction at its best while still being very accessible to a non-sci fi reader. It is a tale truly as old as time: alienation and the other slowly dissolving into understanding, kinship, and friendship.
We are cast into an unknown world just as is our protagonist, Genly Ai, an Envoy of the Ekumen sent to the cold planet Winter to pave the way for an alliance with the 80-odd other planets inhabited by humans. Each world has its own particular brand of human: on Gethen, they are a kind of hermaphrodite. Neuter most of the time, Gethenians go into kemmer about once a month (think an animal in heat) and interaction with other people determines which gender the individual becomes for the sake of mating. Thus can each Gethenian both father and mother children. The impact this has on society is immense, and it is a thread that runs through the book as well as being discussed explicitly a number of times.
"The Left Hand of Darkness" is a journey tale, as well, and a politico-philosophical treatise, and a mythology, and many other things. The writing is superb, descriptive enough to paint a vivid picture without being bogged down. Small wonder this gem won both the Hugo and Nebula awards.