Strindberg's preface to Miss Julie, rather like the play itself, is a manifesto of naturalism in the theater. In very basic terms, this means that it aims to set out to the reader a very different kind of theater from what they're used to. Instead of setting out to entertain his audience, or preach conventional moral platitudes at them, Strindberg's going to present the lives of contemporary characters in terms that are utterly realistic.
Such a naturalist attitude is marked by a notable lack of sentimentality. As Strindberg makes clear in the preface the rise and fall of families such as Miss Julie's form a natural part of life; they are not to be lamented. Strindberg wants his audience to observe the characters on stage, almost as if they are a group of scientists observing animal life. At no point does he want them to establish any kind of meaningful connection with the characters on stage.
This helps to explain why Strindberg chastises his audience for finding his works sad. In order to find a play sad, one has to identify with the characters on stage, to develop empathy with them in common with the conventions of ancient Greek tragedy. But Strindberg aims to depart from those conventions as much as possible. And besides, says Strindberg, there's nothing remotely sad about the decline of upper-class families, who stand in the way of their supposed social inferiors. Anyone who feels sad at Miss Julie, says Strindberg, is guilty of projecting their own feelings onto the action.
Strindberg further attempts to shock his audience by putting forth a very radical notion of what it means to be aristocratic. According to him, there is a natural aristocracy which transcends the rigid hierarchies of society. That being the case, it is possible for Strindberg to describe Jean, who's a servant, as being more naturally aristocratic than Miss Julie, a member of a decaying, decadent artificial aristocracy.
In purely theatrical terms, Strindberg points the way to the future by insisting on the need for realism in staging. Actors will wear little or no make-up, they will be encouraged to turn their backs on the audience at appropriate moments, and footlights will be abolished. All of these innovations exist to heighten the play's naturalism, its rootedness in the rhythms of everyday life.