Le Guin's novel seems to be an interesting, if strange, subject -- bisexual individuals -- yet it seems that a majority of the sort of "oddness" that appears is either random differences from humans -- for example, not being able to say the "L" sound (Le Guin 30) -- or in the developing relationship between Genly and Estraven in the later parts of the novel. Other than that....most of the events and societal customs observed seem either familiar or not very outlandish.
Even the whole gender-neutral aspect seems to be focused on very little...with only random thoughts of Genly to remind a reader that these people are, indeed, alien. At least how I read the novel, these individuals could easily pass as normal people, and the only defining characteristic that marks the world as alien is the endless cold and snow that permeates nearly every page.
From the way it is described for the beginning of the novel, Gethen is not a very interesting place compared to Earth....and could almost be the same. As time goes by however, primarily beginning with Genry and Estraven's journey across the ice, things seem to change. Suddenly the "interesting" qualities of Estraven are more apparent (what with the two of them alone out there) and for probably the first time it is easily apparent both to Genry and us as readers how alien and different such a characteristic (bisexualism) really is. While for most of the novel the various people appear as solely male...Estraven slowly but surely shows his feminine side during the times in the tent on the Gobrin ice, especially when he comes to be in kemmer (232).
After several chapters of stark contrast, the novel ends similarly to how it began....but the effect of that time the pair spent on the ice is still there, and even Genry notices how much the natives have changed him...seeing his own kind as strange (296). As a result, even though the topic was avoided for a majority of the novel, the dual feminism and masculinity of the people of Gethen seems all the stranger...yet much more interesting for the experience.
I don't think you mean to say this, but "not being able to say the L sound" is not a characteristic that marks someone as being nonhuman. In Japan, they have a sound which is somewhere between the R and the L sound. After having spoken only that language for a long time, many Japanese speakers find it extremely difficult to make the same L sound that English-speakers are familiar with.
I agree that the net effect of Le Guin's descriptions of the ambisexual characters is to make them seem strange but almost human. I think that it's very difficult to actually visualize most of the characters in the novel - several are described as (and I don't have the exact quote) "beautiful by male or female standards" but as that doesn't actually tell me much about what they look like, since I have no idea what Le Guin's or Genly Ai's sense of beauty is.
As a matter of fact, I think the book is generally pretty sparse on physical descriptions for the characters. I can only harbor my suspicions regarding how much of that is intentional and how much of that is Le Guin's failure to find the right words or to remember to mention things. I suspect that, to some extent, specific details are omitted to stop us from associating them with a specific gender - but I think it's actually somewhat counterproductive, since we're forced to sort of associate the characters with some image that we already have in mind, in order to visualize them. At least, that's how it worked for me.