The real evils, indeed, of Emma's situation were the power of having rather too much her own way, and a disposition to think a little too well of herself. […] The danger, however, was at present so unperceived, that they did not by any means rank as misfortunes with her. (1.4)
Imagination can be a dangerous thing – especially when it allows an over-inflated ego to take control!
How can Emma imagine she has any thing to learn herself, while Harriet is presenting such a delightful inferiority? (5.15)
Mr. Knightley’s argument is based upon an implicit understanding of the standards of Highbury. As long as no one better than Emma is immediately nearby, she’ll never feel the need to improve herself. Local (not universal) standards are valued.
Vanity working on a weak head, produces every sort of mischief. Nothing so easy as for a young lady to raise her expectations too high. (8.47)
Mr. Knightley, the only true social critic of the novel, argues against Emma’s friendship with Harriet largely because it seems to be based upon a self-deception (on Emma’s part).
[…] she walked on, amusing herself in the consideration of the blunders which often arise from a partial knowledge of circumstances, of the mistakes which people of high pretensions to judgment are for ever falling into […] (13.21)
Emma’s judgment of Mr. Knightley returns to her with a vengeance by the end of the novel!
It was foolish, it was wrong, to take so active a part in bringing any two people together. It was adventuring too far, assuming too much, making light of what ought to be serious, a trick of what ought to be simple. (16.10)
Emma’s moralizing, while accurate, only lasts as long as she remembers to keep her imagination in check.
Emma's very good opinion of Frank Churchill was a little shaken the following day, by hearing that he was gone off to London, merely to have his hair cut. (25.1)
Looks matter in Austen’s novel – but the ability to appear naturally good-looking is akin to being naturally genteel. Too much effort seems gauche. And, in Frank’s case, spendthrift.
[…] certainly silly things do cease to be silly if they are done by sensible people in an impudent way. Wickedness is always wickedness, but folly is not always folly.—It depends upon the character of those who handle it. (26.2)
Frank’s haircut isn’t nearly as troubling as the fact that he doesn’t make a big deal of it. Emma suggests that this could mean he undertakes such "folly" on a regular basis – and is now therefore immune to it.
I would much rather have been merry than wise. (30.18)
Emma’s statement is a bit out of character – she prides herself on her wit. Perhaps Frank’s influence affects her statement here.
Can you trust me with such flatterers?—Does my vain spirit ever tell me I am wrong? (38.47)
Emma’s banter with Knightley reveals that she does, indeed, know how misguided her own reflections can be – even if she refuses to take her conscience seriously.
What right had he to come among us with affection and faith engaged, and with manners so very disengaged? What right had he to endeavour to please, as he certainly did—to distinguish any one young woman with persevering attention, as he certainly did—while he really belonged to another?—How could he tell what mischief he might be doing? (46.45)
Emma accuses Frank of the same recklessness with love which she herself could be guilty of.