What mistakes does Darcy make in his proposal to Elizabeth?

In Chapter Eleven, after inquiring about Elizabeth's health, Mr. Darcy nervously paces around the room for a few minutes. Suddenly, he declares his love for her. He starts by eloquently expressing his admiration. He then refers to the inferiority of Elizabeth's social connections and explains that her family's rather unattractive behavior dissuaded him from proposing sooner. Elizabeth is offended and harshly declines his proposal, much to Darcy's surprise. Elizabeth explains her reasons for turning him down. First, she cites the arrogant manner of his proposal. Second, she explains her distaste over the way he worked to separate Bingley from Jane. Finally, she claims that she could never marry a man who could treat Wickham so badly.

In this scene, Darcy's own pride and prejudice come to the fore. Despite the fact that Elizabeth has never shown him any partiality or affection, she can tell that he has "no doubt of a favorable answer." This is likely because Darcy's immense pride makes it impossible for him to see why Elizabeth might decline the chance to marry into a higher social class. Darcy's resentful reaction to Elizabeth's refusal confirms her suspicion. Furthermore, he makes his strong class prejudices clear in his long speech about the inferiority of her connections. Even worse is the insensitive and tactless manner in which Darcy voices his criticisms. He does not at all consider that his harsh judgement will hurt Elizabeth on a personal level.