The Collaborative International Dictionary of English v.0.48:

Subtlety \Sub"tle*ty\, n.; pl. Subtleties. [OE. sotelte,
   sutilte, OF. sotillete, L. subtilitas. See Subtle, and cf.
   Subtility.]
   1. The quality or state of being subtle, or sly; cunning;
      craftiness; artfulness.
      [1913 Webster]

            The fox which lives by subtlety.      --Shak.
      [1913 Webster]

   2. Nice discernment with delicacy of mental action; nicety of
      discrimination.
      [1913 Webster]

   3. Something that is sly, crafty, or delusive.
      [1913 Webster]

            Unlearned in the world's false subtleties. --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

subtlety
    n 1: a subtle difference in meaning or opinion or attitude;
         "without understanding the finer nuances you can't enjoy
         the humor"; "don't argue about shades of meaning" [syn:
         nuance, nicety, shade, subtlety, refinement]
    2: the quality of being difficult to detect or analyze; "you had
       to admire the subtlety of the distinctions he drew" [syn:
       subtlety, niceness]

Moby Thesaurus II by Grady Ward, 1.0:

240 Moby Thesaurus words for "subtlety":
   Italian hand, absoluteness, accuracy, acquired taste, acuteness,
   airiness, airy nothing, animal cunning, apparent soundness,
   appreciation, appreciation of differences,
   appreciation of excellence, appreciativeness, art, artfulness,
   artifice, artistic judgment, astuteness, attention to detail,
   attention to fact, bodilessness, cageyness, callidity, canniness,
   care for truth, casuistry, choiceness, circularity,
   circumstantiality, civilized taste, civilizedness, cleverness,
   complexity, complexness, complication, connoisseurship,
   conscientiousness, convolution, correctness, crabbedness, craft,
   craftiness, critical niceness, criticality, criticalness,
   cultivated taste, cultivation, culture, cunning, cunningness,
   daintiness, deceptiveness, delicacy, delicate distinction, detail,
   deviousness, differentia, differential, discriminating taste,
   discriminatingness, discrimination, discriminativeness,
   disingenuousness, distinction, distortion, elegance, entanglement,
   equivocalness, equivocation, ethereality, evasive reasoning,
   exactingness, exactitude, exactness, excellence, exquisiteness,
   faithfulness, fallaciousness, fallacy, fastidiousness,
   faultlessness, feel, feeling, fidelity, fine Italian hand,
   fine palate, fine point, fineness, finesse, finicality,
   finicalness, finickiness, finickingness, flawlessness, flimsiness,
   foxiness, fussiness, gamesmanship, good taste, grace, gracefulness,
   gracility, graciosity, graciousness, guile, guilefulness, hairline,
   immateriality, impalpability, imponderability, incorporeality,
   ingeniousness, insidiousness, insincerity, insolidity,
   insubstantiality, intangibility, intricacy, intricateness,
   inventiveness, involution, involvement, jesuitism, jesuitry,
   judiciousness, literalism, literality, literalness, low cunning,
   making distinctions, margin, mathematical precision,
   meticulousness, minuteness, misapplication, mistiness,
   mystification, narrow margin, nice distinction, niceness,
   niceness of distinction, nicety, nuance, obfuscation, obscurantism,
   one-upmanship, oversubtlety, palate, particle of difference,
   particularity, particularness, pawkiness, perfection, perplexity,
   perversion, philosophism, plausibility, plausibleness,
   point of difference, polish, preciseness, precisianism, precision,
   punctiliousness, punctuality, quality, ramification, rareness,
   rarity, rationalization, readiness, refined discrimination,
   refined palate, refinement, resourcefulness, right, rightness,
   rigidity, rigidness, rigor, rigorousness, satanic cunning,
   scrupulosity, scrupulousness, selectiveness, sense, sensibility,
   sensitivity, severity, shade of difference, sharpness, shiftiness,
   shrewdness, slickness, slightness, slipperiness, slyness,
   sneakiness, sophism, sophistical reasoning, sophistication,
   sophistry, special pleading, speciosity, specious reasoning,
   speciousness, stealth, stealthiness, strictness, subtility,
   subtilty, subtle distinction, subtleness, suppleness, tact,
   tactfulness, tanglement, taste, tastefulness, technicality,
   tenuity, tenuousness, textualism, the letter, thinness, treachery,
   trickiness, unconcreteness, unreality, unsolidity,
   unsubstantiality, unsubstantialness, vagueness, vicious circle,
   vicious reasoning, wariness, wide margin, wiles, wiliness, wit