Why would the presentation of Lady Macbeth have been particularly shocking to a 17th century audience?
Lady Macbeth is presented as having characteristics that Shakespeare's audiences would have associated with masculinity. When Lady Macbeth receives Macbeth's letter describing his encounter with the witches, she realizes that she will have to push her husband to murder Duncan, the only way (in her mind) the prophecy can be fulfilled. In one of Shakespeare's most chilling soliloquies, she explicitly assumes a masculine role, asking the "spirits that attend mortal thoughts" to:
...unsex me here
And fill me, from the crown to the toe, top-full
Of direst cruelty! Make thick my blood,
...Come to my woman's breasts,
And take my milk for gall, you murdering ministers,
Wherever in your sightless substances(50)
You wait on nature's mischief!
Later, when her husband vacillates before murdering Duncan, she prods him toward the deed, challenging his masculinity by claiming that "when thou durst do it, then you were a man," and warning him that he will judge himself a coward if he backs out of it. Even after the murder, she calls him weak because he hesitates to go and smear Duncan's blood on his guards.
Lady Macbeth is an example of a theme that Shakespeare works throughout Macbeth, the clash between appearance and reality, or to put it another way, between the natural and the unnatural. Shakespeare's audiences, having different attitudes about gender roles than modern audiences, might have regarded Lady Macbeth as unnatural. The presence of the witches, who actually have beards, underscores the theme. On the other hand, it is worth noting that Shakespeare's audiences had just witnessed the end of more than a half-century of rule by queens (in the form of Mary and Elizabeth), so the image of a powerful woman could have been, for some particularly thoughtful theater-goers, more complex than might be imagined.