You know, Rarity is my favorite pony, and I love her for her flaws as much as for her virtues. Flaws are funnier, after all, and funny characters are more endearing to me than virtuous ones.
Anyway, one of her biggest flaws is linked to her biggest virtue. Her drive to make the world a more beautiful place is a noble one, but it is horribly handicapped by her very narrow view of what qualifies as beautiful. See above screencap for a perfect example.
Personally, I’m just hoping to see her develop a fleeting crush on a mule before the show ends. Now that would be cool bit of character development!