I don't know what it was: perhaps because I'm in a position of trying (imperfectly) to educate young ladies, or because I'm more alive to my own errors, or because I've come to appreciate Mr. Knightly's character in a different light. Austen's books, to me, are all exquisite morality studies (in fine pencil, with very delicate shades of grays), in which characters' ridiculousness, often ugliness, are exposed, which almost always, transparently, trace back to raw egoism.
I think Emma is remarkable partly because of Miss Bates, who is described as being silly and tiresome, and yet with a sweetness of heart which the narrator calls out as making her worth of respect, whereas most of Austen's overtly comic characters are simply ridiculous. There is a tremendous contrast between Miss Bates' repetitive, disconnected effusions, and the horrible pretension and hypocrisy of Mrs. Elton. I also love the contrast between Frank Churchill and Mr. Knightly, where Frank, who enters the scene as a dashing, romantic hero, is revealed to be selfish, occasionally malicious, and immature. Mr. Darcy will probably always be the best of Austen's heroes, but I admire Mr. Knightly more because unlike Mr. Darcy he is generous enough to dance when he does not want to dance.
After finishing the book, I watched my way through all the film versions, and this time, discovered the BBC Masterpiece Classic 2009 mini-series with Romola Garai and Jonny Lee Miller. Sandy Welch, who has adapted a number of period dramas, plays fast and loose at times with the original, but this version absolutely captures the core of the story in a way none of the other versions do. If you haven't seen it, I highly, highly recommend it (available on Netflix). Garai engages and knocks it out of the park. Miller's Mr. Knightly bears a strong resemblance to his Sherlock in Elementary: careful, observant, concerned with truth and right.