Jane's greatest weakness is that she is far too willing to see goodness in people and not see any faults. In this sense, Jane is very naive about the world. We first see Elizabeth accuse Jane of overlooking people's faults when, after meeting Mr. Bingley at the Meryton ball, Elizabeth accuses Jane of liking "many a far stupider person" than Bingley. When Jane protests, Elizabeth further to says,
"Oh! you are a great deal too apt, you know, to like people in general. You never see fault in any body. All the world are good and agreeable in your eyes" (Vol. 1, Ch. 4).
The problem with this fault is that it can steer Jane in the wrong direction, leading her to be hurt. Jane was especially deceived by Miss Bingley's character. While visiting her aunt and uncle in London, when Jane is snubbed by Miss Bingley, Jane is forced to confess that she had been "entirely deceived" in thinking that Miss Bingley thought highly of her and wanted her as a friend (Vol. 2, Ch. 3).
In pointing out Jane's faults in perception, it can be said that Austen is making the point that there were some women in her society who were so poorly educated in the ways of the world that they were as blind and naive as Jane.