Macbeth has mass appeal, and it does not have to pander to its audiences. It is Shakespeare's shortest tragedy, half as long as Hamlet. That appeals to everyone.
It's his bloodiest play, by far. The amount of blood imagery is startling to any audience, any generation. That appeals to everyone.
Macbeth portrays the murder of the King, God's holy vessel, the worst crime imaginable to any audience. I do not agree with the former post that a lower class would sympathize with someone murdering a "boss." Regicide and patricide horrifies everyone, regardless of class.
Most of Shakespeare's contemporaries wrote heavily religious text. And yet, the play can be read from a Christian standpoint. This duality is testament to its mass appeal.
The witchcraft scenes were written for James I, not the illiterate lower classes. Again, the witches are great characters who appeal to everyone, rich and poor, young and old, upper and lower classes.
This is a great question, as Shakespeare often structured his plays to appeal to both the lower and upper classes. He often included bawdy jokes or moments of comic relief for the less educated audience members side by side with the more thought-provoking, philosophical monologues. Even the physical structure of the theaters, circular and multi-tiered as they were, allowed the upper classes to attend along with, yet still separate enough from, the peasants.
In the play Macbeth, the lower classes would probably be thrilled by the witches and the idea of fate that they symbolize. I can imagine that they would also sympathize with Macbeth's desire to do away with his boss. The upper classes would also be interested in Macbeth's crime, though they might be more likely to sympathize with Duncan. They would be very interested in the issues of political power and ambition that affect the Macbeth couple.