Unlike Macbeth, who eventually is feared by his thanes, Duncan is loved and admired by his family and thanes. Duncan is a thoughtful ruler, loyal to his thanes because he feels they are loyal to him. He rewards Macbeth for his loyalty and success on the battlefield. This shows his generosity.
Duncan is perhaps too trusting. And he admits that there is no skill in being able to judge someone's character simply from outward appearances. He could simply be speaking on his own poor judgment. He mentions this about how he mistakenly trusted the original Thane of Cawdor:
There's no art
To find the mind's construction in the face:
He was an absolute gentleman on whom I built
An absolute trust. (I.iv.13-16)
He is extremely kind to Macbeth and Banquo when he meets them after battle. He shows his appreciation to Lady Macbeth for having him at her castle. Duncan is gracious and thankful toward those who are loyal to him. He easily places his trust in others. This shows his loyalty and generosity as a ruler, but it also shows that he is not discerning enough in judging the character of others. In short, he is too trusting. This is in stark contrast with Macbeth who trusts no one.