>>34969305>>Applejack and Rarity>>ZOMG Opposites attract!
They're primarily opposites in two very related things, those being style and social circles they want to be part of. Both work themselves to the bone and want their efforts to benefit others, starting with their immediate surroundings.
AJ's athleticism both ties into the style/social strata thing and is a direct consequence of her trade/talent. She's not even against something nice to wear, she just doesn't want it to be all frilly and shit (i.e. her preferences normally clash with what Rara considers superior but she has preferences to begin with).
RD and Rara have pretty much nothing in common and everything at opposites. RD wants to be a NEET who can occasionally go fast and hang out with ponies she isn't afraid to be seen with. Oh, I guess they have this in common, actually: both eagerly subjugate themselves to societal pressure.