In terms of morality and gender politics, why would one consider Macbeth as one of Shakespeare's most important plays?

The morality in Shakespeare's Macbeth is clear cut and absolute.  The main character and protagonist is clearly evil, or to be more specific, he is clearly, obsessively ambitious and kills or orders the killings of numerous people. 

Contrast this to, for instance, Hamlet. Claudius is clearly evil, but he is not the main character and his guilt is in question through about half of the play.  The tragic figure, however, is not evil and suffers his fall for reasons that do not pertain to morality.

Thus, one might view Macbeth as more imprtant than other Shakespeare plays, in terms of morality.

Concerning gender roles, the reversal of them is central to Macbeth.  The witches appear to be women but have beards; Macbeth, at least early in the play, exhibits emotions more often associated with women--guilt, mercy, doubt; Lady Macbeth, in contrast, wants to be more like a man:  she pleads with the spirits to "unsex" her, turn her figuratively into a man, stop her from feeling emotions like mercy and kindness.  These role reversals make the play important in terms of gender roles.