Roman Polanski chooses to stage the murder of Duncan, something that Shakespeare wrote in Macbeth as offstage action. Not only do we see the murder of Duncan, which is vicious and leaves no doubt that Macbeth is an evil, violent man, which I don't think was Shakespeare's intention, but we see a violent death for Banquo and a grisly death of the first Thane of Cawdor also.
Roman Polanski also eliminates a great deal of the dialogue, cutting it down substantially throughout the production.
We see Macbeth crowned King in the movie, also Polanski adds a dream scene for Macbeth, where he envisions Fleance trying to steal his crown, and then tries to kill Macbeth with the help of his father, Banquo.
Polanski takes away the subtlety of Shakespeare's work by hitting us over the head with violence.
The film is very dark, and does not employ the colorful imagery in the play, just plays up the violence.