On re-reading this classic recently, I was very struck by the clever use of language.
“The Moon is a Harsh Mistress” was first published in 1966, the setting is 2075-6. Heinlein gives his viewpoint character a plausible future Moon dialect of English; a number of Russian words and even some Australian ones such as “cobber”. The character’s speech often lacks the article “the”. I think that is a common characteristic of Russian native speakers because their language is heavily inflected and therefore does not require the constant use of the definite article. So the dialect is plausible in a small future Moon population composed of descendants from many different countries.
The novel launches straight in to a dialogue between this character and a self-aware computer. This beginning was challenging but I found it intriguing. Typically for Heinlein, there is little explanation or description; we learn about the character’s world through watching him living in it.
It is often said that Heinlein’s writing style is so invisible as to be a non-style; I would rather say that the characteristics of Heinlein’s writing are sufficiently distinctive as to constitute a “style”. This kind of discussion often becomes an argument about definitions, but I would say that style is what makes a writer unique, rather than just a competent craftsperson. Did anyone else write like Heinlein? No. You can tell it’s him immediately.
Look at http://templetongate.net/rahstyle.htm for further discussion on this point. Among other things, I agree with the comparison with another genre master, Dashiell Hammett.