Some of the main characters of The Moonstone, including the first narrator, Gabriel Betteredge, are servants. This is really unusual for a Victorian novel. In most novels of this time period, servants exist almost as props – they're background characters, and that's it. But in The Moonstone, they are not only important to the story, they actually speak up and talk about their social position. Betteredge and Rosanna Spearman both make some very politically charged comments about social class and the position of servants.
The Moonstone is unique not just because some of the main characters are servants, but because those characters are portrayed sympathetically.
Wilkie Collins actually makes a profound political commentary through the narration of Gabriel Betteredge, who criticizes the tendency of the wealthy to create messy, pointless hobbies to fill the void left by their lack of productive labor.