In the very beginning of the novel, immediately after the ball at Meryton, Elizabeth tells Jane her opinion that Jane is "a great deal too apt ... to like people in general" (Ch. 4). A central theme in the book is the ability to judge rightly. As the story progresses, we learn that Elizabeth, who has "prided [herself] in [her] discernment," meaning her ability to see and judge correctly, is actually a bit blind (Vol. 2, Ch. 36). She far too hastily judges Darcy to be conceited, prideful, and arrogant, simply because he is reserved, and far too hasty to judge Wickham as the best man she's ever met, simply because he is cheerful, has good manners, and is able to converse warmly and freely.
As Elizabeth points out in Chapter 4, in contrast to Elizabeth, Jane is a bit too naive in her desires to see the good in anyone and to admire everyone. As Jane retorts, "I would wish not to be hasty in censuring any one" (Ch. 4). But as Austen points out through Elizabeth, it is as equally ridiculous to be "honestly blind to the follies and nonsense of others," meaning blind to and ignorant of a person's faults, as it is to be too hasty to observe faults. Instead, a happy medium must be reached. One needs the ability to both correctly judge virtues as well as vices.
Hence, in this chapter, Elizabeth accuses Jane of liking people far too easily, or being ignorant of people's character flaws, and even of taking every one's good character traits and making them "still better" (Ch. 4).