The biggest interest for me is the portrayal of her early Indo-European culture, the rustic and earthy precursors to the sophisticated religious and civic institutions of Rome. We hear a few words of pre-Latin for example. The beliefs and rituals emerge from the business of everyday life, which we can extrapolate onto urban Roman culture, and onwards to European culture in general. Le Guin does not minimise the martial and patriarchal nature of the precursor society. It reminds me of the Ancient Greek stories of Mary Renault.
Here is part of a discussion Lavinia has with the spirit of Virgil, when he explains that Juno opposed Aeneas because he was the protegee of Venus.
I pondered this. A woman has her Juno, just as a man has his Genius: they are the names for the sacred power, the divine spark we each of us have in us. My Juno can't 'get into' me, it is already my deepest self. The poet was speaking of Juno as if it were a person, a woman, with likes and dislikes: a jealous woman.
The commentary is on the development of naturalistic religion into a formalised narrative mythology. I think the pairing of 'Juno' with 'Genius' is very interesting. Compare 'Hera' and 'Hero' - in both cases as mythology develops into formal religion, the female active principle is externalised as a goddess, and finally discarded altogether, while the male equivalent is embodied in living men, and continues to be real. Heroes and Geniuses exist right now.
And here's a longer etymological digression - I have long thought that 'Janus' and 'Jana' (ie 'Diana') are male and female forms of the same pre-Indo-European term - the basque word for 'god' is Jainko. You could argue that 'Genius' and 'Juno' represent a further development of the same term.
Anyway, that's my purely speculative train of thought which has strayed quite a way from the novel. I think the interest of the novel rests most strongly in the way it constantly suggests the forms of Roman society, and hence of modern ideas, in a world which is not unlike the Valley of the Na. I think that Le Guin is demonstrating (or at least arguing) the continuity between her vision and modern western culture - I see a lot of criticism of Le Guin which says she paints a dream world which has nothing to do with what people are 'really like' (hard headed, mercantile, avaricious, expansive: ie, modern western people). I think she is making a case here for the relevance of her vision.