Although Duncan is depicted as a benevolent, compassionate king, he is a poor judge of character and too trusting. At the beginning of the play, the Thane of Cawdor takes advantage of Duncan's trust by joining forces with the King of Norway. Fortunately, Macbeth and Banquo are able to defeat the Norwegian forces and the Thane of Cawdor is captured and executed.
In act 1, scene 4, King Duncan comments on his inability to recognize a person's true intentions by saying,
There’s no art
To find the mind’s construction in the face.
He was a gentleman on whom I built
An absolute trust. (1.4.13–16).
King Duncan openly admits that he is a poor judge of character by placing his "absolute trust" in a traitor. King Duncan's inability to recognize the Thane of Cawdor as a dangerous enemy foreshadows his mistake in trusting Macbeth.
After Macbeth is given the title Thane of Cawdor and the first prophecy is confirmed, he entertains the idea of murdering King Duncan and begins planning the assassination with his ambitious wife. Unfortunately, King Duncan once again places his trust in the wrong people. The audience recognizes that Macbeth and his wife are Duncan's worst enemies and currently plotting his demise. When King Duncan arrives at Macbeth's castle, he demonstrates his poor judge of character by referring to Lady Macbeth as a "fair and noble hostess" and is excited about the possibility of visiting their estate. Later that night, Lady Macbeth drugs Duncan's chamberlains and her husband assassinates the king.
Overall, King Duncan's major shortcoming is his poor judge of character. King Duncan is entirely too trusting and does not recognize his true enemies.