Mary had neither genius nor taste; and though vanity had given her application, it had given her likewise a pedantic air and conceited manner, which would have injured a higher degree of excellence than she had reached.

I have never quite understood that line. Mary plays better than her sister, and I do understand how her manner would injure the performance, but I don't understand how it would have "injured a higher degree of excellence" more than her present ability. Generally the reverse is true; we are more willing to excuse a degree of conceit in an excellent performer that might provoke snickers in the less talented.

What is it I'm missing? Is this simply JA's wit? Is there perhaps a parallel between Bingley and Darcy? Elizabeth, easy and unaffected, had been listened to with much more pleasure. In chapter 3 Bingley is described as having easy, unaffected manners. As loathe as I am to compare Darcy to Mary, her "conceited manner" parallels Darcy's conceit; the difference is that Darcy is perhaps expected by Meryton society to have achieved a higher degree of excellence in the social graces, and as a result, he gives offense. Mary has not yet created that expectation.

It's an interesting parallel, although I'm still not sure it explains the line I put in boldface.

(I know we're moving on but wanted to slip this in before we are irrevocably past the first chapters)