In Macbeth, which trait of her husband's does Lady Macbeth fear will prevent him from securing the Scottish throne?
In Shakespeare's Macbeth, Lady Macbeth knows her husband possesses the ambition to want the Scottish throne, but she is afraid he does not possess the personality traits necessary to go through with what it takes to achieve it (kill Duncan). In a famous speech she worries that:
It is too full o'th'milk of human kindness
To catch the nearest way [assassinate the king]. (I:5)
Thus, she worries that he is too kind. She also worries that he doesn't possess the "illness" (evil) to do it. He would rather do it "holily" than to use deceit.
In this speech she also introduces an intricate personality trait of her husband's. She says that he won't want to do it, even though, once it's done, he won't "wishest [it] should be undone." So it's not necessarily the event that bothers Macbeth, it's the doing it himself.
When Lady Macbeth finds out that her husband is thinking of becoming king, she thinks it is a great idea. But she worries that he will not be able to actually accomplish that.
The reason is that she thinks he lacks the drive to match his ambition. She thinks that he has enough ambition, but she worries that he will not follow through. She also thinks that he is not ruthless enough to do what needs to be done.
The best place to see this is in Act I, Scene 5.