It's all about conversations in Stranger in a Strange Land. There's lots of dialogue and generally small sentences that build concise paragraphs. Partly, this is because your average chapter consists of people entering a room, sitting down, and talking about whatever's happened. But it's also partly due to Heinlein's sharp, conversation-like sentences. Just check out the following example courtesy of, who else, Jubal:
But what was "Man"? A featherless biped? God's image? Or a fortuitous result of "survival of the fittest" in a circular definition? The heir of death and taxes? The Martians seemed to have defeated death, and they seemed not to have money-property, nor government in any human sense—so how could they have taxes? (14.155)
The short questions give the vibe of an old man sitting bar-side and throwing his unedited thoughts to the bartender. They also help develop that wry humor we talk about in the "Tone" section. We mean, really, Jubal has just learned that there is an alien race that has become effectively immortal, but he's more worried about taxes?
Stranger in a Strange Land is educational and philosophizing, right? But the writing style is not. And that's what keeps it accessible and makes it a classic. With academic writing, it's easy to get the feeling the writer is trying to hide weak ideas behind long, complex, untranslatable sentences—just like a magician hides a wire behind smoke and a beautiful assistant. With Stranger, the sentences invite you to explore the ideas without the need for sentence showmanship.