The event had every promise of happiness for her friend. Mr. Weston was a man of unexceptionable character, easy fortune, suitable age and pleasant manners; and there was some satisfaction in considering with what self-denying, generous friendship she had always wished and promoted the match; but it was a black morning's work for her. (Chapter 1)

I think the main difference between the Westons and Coles is that Emma likes one and does not the other. Emma truly cares for Miss Taylor because of their past relationship but the first description of Mr. Weston in Chapter 1, which seems to be Emma’s point of view, is not a particularly glorious one. When reading it I wondered why Emma promoted a match between her beloved Miss Taylor and a man she thinks of as unexceptional but when Emma was matchmaking Mr. Elton for Harriet there are several passages where she notes his character flaws, self-centeredness was one I think but thought he was good enough for Harriet although not good enough for herself. I think without a Mrs. Weston the mister would loose some of his gloss.

I think other important considerations for Emma are the virtues of beauty, elegance and taste. Emma is attracted to Harriet in Chapter 3 because of her beauty and her good taste which is illustrated by an eagerness to look up to Emma. Fortunately for Harriet she is still pretty because I think Emma has come to realize Harriet’s taste is not much improved by Chapter 36. Emma describes the Cox daughters as the most vulgar girls in Highbury, probably meaning they have no taste and no beauty but in reality despite being the daughters and sisters of lawyers they are a step up from Harriet who is illegitimate.

Some people seem to put Emma at war with herself; snobbery versus virtues—her like and dislike usually makes the difference in how she views them. Emma persuades herself into cordial feelings about Jane Fairfax for a few paragraphs in Chapter 20 because she is elegant, tasteful and accomplished. By the end of Emma’s initial visit however her dislike has overcome Jane’s virtues and Emma cherishes her suspicions of Jane once again. Emma deplores Frank’s indifference to a confusion of rank but because she likes him rationalizes he is no judge of the evil he is holding cheap considering it only an effusion of lively spirits. Mr. Martin and his family have gone from vulgar people doing Harriet harm in Chapter 3 to folks who if they were just a little higher in situation would be right for Harriet in Chapter 23. Emma has slowly accepted they are not the vulgar illiterate people she thought they were but she still believes them not quite deserving of Harriet IMO because Emma is just not willing to give Harriet up to gentleman farmer.

I found Emma’s decision to pursue comfort for herself after the visit to the Martins shocking because Emma’s scruples in chapters 22 and 23 convinced me for a moment, perhaps finally, she understood slighting the Martins was not worth the guilt she felt for doing wrong and the pain she caused Harriet and them. I agree with Rachel G that commiserating about her own pain after giving pain to others is completely self-centered but perhaps there is some improvement. At least she did have scruples at her project from the start although she did not listen to her conscious and second, the news of Frank did not completely drive thoughts of what she did to the Martins from her head as Mr. Elton’s appearance on Vicarage-lane and her matchmaking in Chapter 10 drove all thoughts of the poor cottagers out of her head:

“But neither geography nor tranquility could come all at once, and Emma was now in a humour to resolve that they should both come in time.” (Chapter 23)