Kind of what the Left Hand of Darkness is about, "the left hand of darkness is the light" ie. you can't have one thing without the other, so the genderless people are neither gender... and both. Or at least, that's how I interpreted it. I expect lots of people can interpret it lots of different ways! As for the characters seeming a bit flat... I sort of felt like the only one we got to spend any time with was Estraven. The king, though, was a pretty colourful character, even if they aren't very nice

I'd have enjoyed seeing more of them.
Haven't read the Dispossessed, I did read Lathe of Heaven and loved it, but also read A Wizard of Earthsea and found it a bit meh. Not bad, just... not that great.
I'm working on a scifi novel where all the aliens are gender neutral (in a different way to Left Hand, I made it as different as I could so it wouldn't seem like a copycat thing!) and this thread has me itching to get working on it again!