Duncan's rule is awesome. People love the guy. They honor and respect him as king. They do this, #1, because he is the blood lineage king. He's supposed to be there. The people consider him God's appointed servant.
"The Lord's anointed temple, and stole thence
The life o' the building!"
That kind of anointing though is true of good kings and bad. Duncan is a respected and liked king, because of his goodness. In the opening act of the play, the audience is introduced to King Duncan. He comes across as a magnanimous figure. The wounded soldier comes up to Duncan, and Duncan is full of praise for the man's bravery and work. Duncan appears to genuinely care for his subjects.
"So well thy words become thee as thy wounds;
They smack of honour both. Go get him surgeons."
Macbeth on the other hand comes to the throne through devious means. He kills the king to become king. Then he kills anybody else that might lay a claim to the throne. Macbeth is so feared that Malcolm and Macduff have to flee Scotland and plot with the King of England to find a way to remove Macbeth from the the throne. Macbeth is a tyrant king. Duncan is a fatherly king. They stand at completely opposite sides of the spectrum.