You are meant to fetishize the entire body of a woman, and appreciate the subtleties of her form. Feet however are by far the most complex part geometrically, with the most intricate shapes, very sensitive and connected in the brain to sensuality. Also, since they are usually covered they are naturally erotic (just ask the victorians), especially with their phermononic odor.
It's a musty, damp smell, fleshy and a bit salty
Smells like wet textiles or used leather if she was in her shoes for a long time, and usually you can smell the pheromones. The usual variation is between a sharper, sourish or a rougher, earthy/grainy variation to the above.
Female foot odor is different than male foot odor, female foor odor is rife with "good" sexual pheromones which drives a male wild. If she gets into him and he pauses ever so slighly, she is going to mind control him. Not only will he be servicing her vagina with his mouth and penis if she desires both, those same feet that were supposed to be gross are going to be licked clean. The unwashed female scent can mindcontrol males, but usually not the other way around.
Done right, feet aren't "just" a fetish.