What information are we given which rouses our immediate interest in Macbeth?

Shakespeare's Macbeth opens with an eerie scene in which three witches are chanting and foreshadowing a later meeting with our title character, Macbeth. After the battle in which Macbeth is fighting, they will meet him on the heath. This sets our expectations; we want to know what will happen when Macbeth encounters these characters. Further, since this is the opening scene, it creates an ominous mood for the play. We can imagine that what will come next will be tragic, or at least strange.
In the third scene of the play, Macbeth and Banquo do, in fact, meet the witches, who appear out of nowhere on the heath. The witches hail Macbeth as "Thane of Glamis," "Thane of Cawdor," and "King hereafter." He is already Thane of Glamis, and he will be named Thane of Cawdor in the next scene because the former thane betrayed the king. When he hears he will be king one day, Macbeth wonders if he should act on the prophesy or let it happen, but he already is considering killing King Duncan to gain the position. This very early suggestion of the murder and treachery to come pique our interest and make us want to keep watching or reading the play.