WordNet (r) 3.0 (2006):

soft spot
    n 1: a place of especial vulnerability [syn: weak part, weak
         spot, soft spot] [ant: good part]
    2: a sentimental affection; "she had a soft spot for her
       youngest son"
    3: any membranous gap between the bones of the cranium in an
       infant or fetus [syn: fontanelle, fontanel, soft spot]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "soft spot":
   a thing for, affinity, aptitude, aptness, assailability, bent,
   bias, cast, chink, conatus, conduciveness, delight, diathesis,
   disposition, eagerness, exposed nerve, feeling for, fondness,
   heel of Achilles, inclination, leaning, liability, liking,
   loophole, nerve ending, penchant, penetrability, predilection,
   predisposition, pregnability, prejudice, probability, proclivity,
   proneness, propensity, raw, raw nerve, readiness, sensitivity to,
   sore point, sore spot, susceptibility, taste, tendency,
   tender spot, the quick, tropism, turn, twist, vincibility,
   vulnerability, vulnerable point, warp, weak link, weak point,
   weakness, willingness