Her skill of reading someone's feelings (empathy) is one of her greatest strengths. I think the first time we've seen a glimpse of that is in Sonic Rainboom. You know, when she bumps Twilight to find a spell? pretty unf scene btw
She often shows a great amount of understanding where the others are oblivious. Rarity can read a pony like nobody else can, and that's great. It's not something that is shown in your face, often it's just a small hint here and there, a gesture or expression, but it's consistent and I love her for it.
Sometimes it actually is for the worse - like in RTM where she reads too much into her friends' displeasure with working instead of seeing the musical, and breaks under the stress. Not that she wasn't right.
Speaking of RTM, at the end when Coco gives her the trophy, Rarity gives that sighing hum when offering Coco a job, and I love that small detail. Often the dialogue is too clean cut, just say your lines, laugh or sigh when told to and make it obvious. This one feels subtle, so natural, so genuine.
I get delighted by the smallest things.