The Collaborative International Dictionary of English v.0.48:

Dullness \Dull"ness\, n.
   The state of being dull; slowness; stupidity; heaviness;
   drowsiness; bluntness; obtuseness; dimness; want of luster;
   want of vividness, or of brightness. [Written also
   dulness.]
   [1913 Webster]

         And gentle dullness ever loves a joke.   --Pope.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dullness
    n 1: the quality of being slow to understand [syn: dullness,
         obtuseness]
    2: the quality of lacking interestingness; "the stories were of
       a dullness to bring a buffalo to its knees"
    3: a lack of visual brightness; "the brightness of the orange
       sky was reflected in the dullness of the orange sea" [ant:
       brightness]
    4: lack of sensibility; "there was a dullness in his heart";
       "without him the dullness of her life crept into her work no
       matter how she tried to compartmentalize it."
    5: without sharpness or clearness of edge or point; "the
       dullness of the pencil made his writing illegible" [syn:
       dullness, bluntness] [ant: asperity, keenness,
       sharpness]

Moby Thesaurus II by Grady Ward, 1.0:

347 Moby Thesaurus words for "dullness":
   abruptness, absence of color, acedia, achromaticity, achromatism,
   adynamia, aloofness, analgesia, anemia, anesthesia, angst, anguish,
   anxiety, apathy, aridity, aridness, ashiness, asininity, ataraxia,
   ataraxy, atony, autism, barrenness, benumbedness, blah,
   blah feeling, blahs, blockishness, bloodlessness, bluffness,
   bluntness, boobishness, boorishness, boredom, boringness, bovinity,
   cachexia, cachexy, cadaverousness, callousness, canescence,
   catatonia, cheerlessness, chill, chilliness, chloranemia,
   cloddishness, cold blood, cold heart, coldness, colorlessness,
   coma, comatoseness, commonness, commonplaceness, coolness,
   cowardice, crassness, crudeness, deadness, deadpan,
   dearth of ideas, deathly hue, deathly pallor, debilitation,
   debility, denseness, density, detachment, dim-wittedness, dimness,
   discomfort, discomposure, discontent, disinterest, dislike,
   dispassion, dispassionateness, displeasure, disquiet,
   dissatisfaction, doltishness, drabness, dread, drowsiness, dryness,
   dull-wittedness, dullardism, dumbness, duncery, easy temper,
   electronarcosis, emotional deadness, emotionlessness, emptiness,
   enervation, ennui, etiolation, even temper, existential woe,
   exsanguination, fadedness, faintness, fairishness, fairness,
   fatigue, feebleness, flabbiness, flaccidity, flatness, frigidity,
   frostiness, ghastliness, glaucescence, glaucousness, good temper,
   gray, grayishness, grayness, grimness, grossness, haggardness,
   heartlessness, heaviness, hebetude, hopelessness, huelessness,
   humdrumminess, humdrumness, hypochromia, hypochromic anemia,
   iciness, immovability, impassibility, impassiveness, impassivity,
   imperception, imperceptiveness, imperceptivity, impercipience,
   imperturbability, imperturbableness, impotence, inanimation,
   inappetence, inconsiderateness, indifference, inexcitability,
   inexcitableness, infecundity, infertility, inirritability,
   inquietude, insensibility, insensibleness, insensitiveness,
   insensitivity, insentience, insipidness, insouciance, irksomeness,
   jadedness, joylessness, lack of affect, lack of appetite,
   lack of bite, lack of feeling, lack of pleasure, lack of touch,
   lackadaisicalness, lackluster, languidness, languishment, languor,
   languorousness, lassitude, leadenness, lenitude, lentor,
   lethargicalness, lethargy, lifelessness, lightness, listlessness,
   lividity, lividness, long-windedness, loutishness, lumpishness,
   malaise, matter-of-factness, mediocreness, mediocrity,
   middlingness, moderateness, modestness, modesty, monotonousness,
   mousiness, muddiness, muffled tone, mutedness, narcosis,
   narcotization, nausea, neutral tint, ninnyism, nonchalance,
   nongratification, nonsatisfaction, numbness, oafdom, oafishness,
   objectivity, obtundity, obtuseness, opacity, oscitancy,
   painfulness, paleness, pallidity, pallidness, pallor, passableness,
   passionlessness, passiveness, passivity, pastiness, patience,
   phlegm, phlegmaticalness, phlegmaticness, pins and needles,
   plainness, plucklessness, poker face, prison pallor, prolixity,
   prosaicism, prosaicness, prosaism, prosiness, prostration,
   resignation, resignedness, respectability, sallowness, satedness,
   savorlessness, self-absorption, sickliness, sickly hue,
   silveriness, simpletonianism, slatiness, sleep, sleepiness, sloth,
   slothfulness, slowness, sluggishness, slumber, smokiness,
   smooth temper, soberness, softness, somberness, somnolence, sopor,
   soporifousness, sottishness, soullessness, spiritlessness, spleen,
   spunklessness, staidness, staleness, steadiness, stoicism,
   stolidity, straight face, strengthlessness, stuffiness,
   stupefaction, stupidity, stupor, supineness, tastelessness,
   tediousness, tedium, thick skin, thick-headedness,
   thick-wittedness, tiresomeness, tolerableness, tonelessness,
   toothlessness, torpidity, torpidness, torpitude, torpor,
   uncomfortableness, unconcern, uncreativeness, unease, uneasiness,
   unembellishedness, unemotionalism, unexcitability, unfancifulness,
   unfeeling, unfeelingness, unhappiness, unidealism,
   unimaginativeness, unimpressibility, unimpressionableness,
   unirritableness, unnervousness, unoriginality, unpassionateness,
   unperceptiveness, unpleasure, unpoeticalness, unresponsiveness,
   unromanticism, unsatisfaction, unsharpness, unsusceptibility,
   unsympatheticness, unteachability, untouchability, vapidity,
   veiled voice, vexation of spirit, voce velata, wanness, weakliness,
   weakness, wearifulness, weariness, wearisomeness, withdrawal,
   withdrawnness, world-weariness, wrongheadedness, yokelism