Emma’s relationship (or lack thereof) with Mr. Elton demonstrates a great deal of how social hierarchy works in the novel as a theme. Because of Emma’s dallying with Harriet and Mr. Elton she encourages shifts in the social order by having Harriet turn down a suitable proposal from Mr. Martin. Then, because of her meddling, Harriet begins to develop feelings for Mr. Knightley. As the story progresses we see a continuation of this through Mr. Elton. In much the same way that Emma wanted to make a better match for Harriet, so Mr. Elton wanted to make a better match for himself.
Eventually when Mr. Elton does chose a wife he considers to be worthy of him, she is remarkably unlikable. This can be seen as a warning against the dangers of marrying for money or status, as it seems the only happy marriages in Austen novels are the ones that are created out of affection.