I concur with Glenn that these chapters of this book are my absolute favorite in ALL of Emma and indeed ALL that Miss Austen has written. Some random and rambling thoughts:

Romantic love blooming?
Mr. Knightley dancing with both Harriet and Emma. It was obvious that Mr. Knightley and Emmer had NEVER danced together and Emma probably had never seen Mr. Knightley dance at all. I think that was a powerful incentive and Mr. Knightley's transition from familial love to romantic desire.

Although Emma visits Miss Bates, her mother and Jane the very next morning following the mortifying insult, Emma is contrite but I never did see anything approaching an actual apology. Perhaps it is beneath Emma's station to condescend to apologize. Nevertheless, I admire Emma's wish to make amends however she might.

From Ch 45 - More romance?
Mr. Knightley raising Emma's hand to kiss it and then letting it back down. I found this act full of subtle eroticism. Perhaps Mr. Knightley refrained from completing the kiss being in the presence of Mr. Woodhouse and might have thought it inappropriate. If there was no one around, do you think Mr. Knightley would have found it irresistable to sweep Emma into his arms in a loving embrace at that moment? I think he was feeling VERY much at this time for Emma.

From Ch 48 - very-telling:

"....She had herself been first with him for many years past. She had not deserved it; she had often been negligent or perverse, slighting his advice, or even wilfully opposing him, insensible of half his merits, and quarrelling with him because he would not acknowledge her false and insolent estimate of her own -- but still, from family attachment and habit, and thorough excellence of mind, he had loved her, and watched over her from a girl, with an endeavour to improve her, and an anxiety for her doing right, which no other creature had at all shared. In spite of all her faults, she knew she was dear to him; might she not say, very dear?......"
She realizes what a singularly nurturing and benevolant force Mr. Knightley has been on her entire life at this moment. It sort of reminds me of Fanny and Edmund how he was such an influence on her from a young age.

I believe Jane Fairfax's surreptitious engagement with Frank Churchill was the cause for very reserved and closed behavior. I cannot otherwise explain the attachment between these two seemingly opposite personalities. I think Miss Fairfax is not nearly so cautious, reserved, or closed once the engagement is revealed. This should come out in the latter chapters of the story.

Emma's self-realization become almost full-blown in Ch 48, other than her resolution to still "never marry." As she reflects on her miserable failings she finally "gets it."

Ch 49 - More "eros:" Mr. Knightley presses Emma's hand next to his heart. I don't know quite how he did that but that was just about as far as a gentleman could go in touching a woman in that manner.

Finally, the crowning glory of the entire story for me is the turn in the bushes between Emma and Mr. Knightley upon his return from London. Personally, at this point in the story on my very first reading, I was so intimately involved in the characters, ESPECIALLY with Emma and Mr. Knightley that I found myself audibly coaxing Mr. Knightley to "GOOD GOD, GO AHEAD AND ASK HER!" When he finally FINALLY did, I jumped up and pranced around the room with exultation! It was just the most sublime encounter and most feelingly romantic dialogue I have ever read.

The angst and apprehension of Mr. Knightley was almost overpowering.

In Ch 49 Miss Austen again exhibits the mental acuity of Emma's mind:

"....While he spoke, Emma's mind was most busy, and, with all the wonderful velocity of thought, had been able -- and yet without losing a word -- to catch and comprehend the exact truth of the whole....."

And finally, from Ch 49, we get another stellar glimpse into the heart of Mr. Knightley and his ability to love unconditionally, in spite of Emma's faults and weaknesses:

".....so much keen anxiety for her, that he could stay no longer. He had ridden home through the rain; and had walked up directly after dinner, to see how this sweetest and best of all creatures, faultless in spite of all her faults, bore the discovery....."