WordNet (r) 3.0 (2006):

weak point
    n 1: an attribute that is inadequate or deficient [ant: forte,
         long suit, metier, speciality, specialty,
         strength, strong point, strong suit]

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "weak point":
   argument, argument by analogy, argumentum ad baculum,
   argumentum ad captandum, argumentum ad hominem, assailability,
   bad case, bad habit, begging the question, besetting sin, blemish,
   bug, catch, chink, circular argument, claptrap, crack,
   crowd-pleasing argument, defect, defection, deficiency, drawback,
   empty words, failing, failure, fallacy, fault, faute, feet of clay,
   flaw, foible, formal fallacy, frailty, heel of Achilles, hole,
   hysteron proteron, imperfection, inadequacy, infirmity,
   insincere argument, kink, little problem, logical fallacy,
   material fallacy, mere rhetoric, moonshine, moral flaw,
   non sequitur, paralogism, penetrability, petitio principii,
   philosophism, pregnability, problem, pseudosyllogism, rift,
   shortcoming, snag, soft spot, solecism, something missing, sophism,
   sophistry, taint, verbal fallacy, vice, vincibility, vulnerability,
   vulnerable place, vulnerable point, weak link, weak side,
   weakness