In a thread below (Feeling Cheated, #45900) Cheryl laments the lack of details in the proposal scene that we get in Ch 58. JA did provide plenty of specific dialog in the previous two proposals, but this is all brevity and Elizabeth's reply is in indirect speech.

Elizabeth was too much embarrassed to say a word. After a short pause, her companion added, "You are too generous to trifle with me. If your feelings are still what they were last April, tell me so at once. My affections and wishes are unchanged; but one word from you will silence me on this subject for ever."

Elizabeth, feeling all the more than common awkwardness and anxiety of his situation, now forced herself to speak; and immediately, though not very fluently, gave him to understand that her sentiments had undergone so material a change since the period to which he alluded, as to make her receive with gratitude and pleasure his present assurances.

Richard Jenkyns, in a chapter in "A Fine Brush on Ivory," has this to say, and I found it very compelling: "The brevity is fitting because the decision, as we know, has already been made; and the indirect speech is fitting because Jane Austen is writing subjectively, seeing events through the medium of the heroine's consciousness; the vagueness of her account matches Elizabeth's confusion, form agreeing with content. On the earlier occassions...she may have been amused, indignant, or angry, but she has remained essentially master of herself: she has been able to hear herself. Now, at this moment of supreme embarrassment and happiness, she can no longer hear herself, and so we cannot hear her either."

I find Jenkyns' analysis of why we are not privy to more details makes great sense. If you look at what immediately follows in this light, I think you can see that Elizabeth remains somewhat embarrassed and perhaps overwhelmed by what has just happened, when she had in fact despaired of it. So for example, she knows Darcy has expressed himself "as sensibly and as warmly as a man violently in love can be supposed to do" but she may not be able in the future to recall exactly what he said. She cannot even bring herself to encounter his gaze. In fact, they don't even notice where they are going!

The happiness which this reply produced was such as he had probably never felt before, and he expressed himself on the occasion as sensibly and as warmly as a man violently in love can be supposed to do. Had Elizabeth been able to encounter his eyes, she might have seen how well the expression of heartfelt delight diffused over his face became him; but, though she could not look, she could listen, and he told her of feelings which, in proving of what importance she was to him, made his affection every moment more valuable.

They walked on, without knowing in what direction. There was too much to be thought, and felt, and said, for attention to any other objects.