The Collaborative International Dictionary of English v.0.48:

Plainness \Plain"ness\, n.
   The quality or state of being plain.
   [1913 Webster]

WordNet (r) 3.0 (2006):

plainness
    n 1: the state of being unmixed with other material; "the
         plainness of vanilla ice cream"
    2: clarity as a consequence of being perspicuous [syn:
       perspicuity, perspicuousness, plainness]
    3: the appearance of being plain and unpretentious
    4: an appearance that is not attractive or beautiful; "fine
       clothes could not conceal the girl's homeliness" [syn:
       homeliness, plainness]

Moby Thesaurus II by Grady Ward, 1.0:

252 Moby Thesaurus words for "plainness":
   Atticism, Bohemianism, accessibility, affability, apparentness,
   approachability, appropriateness, artlessness,
   atmospheric visibility, austerity, baldness, bareness, bluffness,
   bluntness, broadness, brusqueness, cacophony, candidness, candor,
   casualness, ceiling, ceiling unlimited, chasteness, chastity,
   childlikeness, clarity, classicalism, classicism, clear-cutness,
   clearness, clumsiness, coherence, comeliness, common speech,
   common touch, commonness, commonplaceness, communicativeness,
   connectedness, consistency, conspicuity, conspicuousness,
   conversableness, cordiality, correctness, crystal-clearness,
   crystallinity, defacement, definiteness, definition, dignity,
   directness, discrimination, disfigurement, distinction,
   distinctness, dullness, dysphemism, ease, easiness, easygoingness,
   elegance, elegancy, elementarity, evidence, evidentness,
   explicitness, extroversion, familiarity, felicitousness, felicity,
   finish, fittingness, flatness, flow, flowing periods, fluency,
   folksiness, forthrightness, frankness, freedom, freeness,
   fundamentality, genuineness, good taste, grace, gracefulness,
   gracelessness, gracility, graciousness, guilelessness,
   high visibility, homeliness, homespun, homeyness, homogeneity,
   household words, humbleness, humility, ill-favoredness, inelegance,
   informality, informalness, ingenuousness, ingloriousness,
   innocence, innocuousness, insipidness, integrity, irregularity,
   leanness, limpidity, looseness, low visibility, lowlihood,
   lowliness, lucidity, manifestness, matter of fact,
   matter-of-factness, meanness, meekness, modesty, monism, naiveness,
   naivete, naivety, naturalness, neatness, obviousness,
   offhandedness, oneness, openheartedness, openness,
   openness to sight, outgoingness, outspokenness, overtness,
   palpability, patentness, pellucidity, percipi, perspicuity,
   plain English, plain dealing, plain speaking, plain speech,
   plain style, plain words, plainspokenness, polish, poorness,
   prominence, propriety, prosaicism, prosaicness, prosaism, prose,
   prosiness, purity, refinement, relaxedness, restrainedness,
   restraint, roundness, rustic style, seeing, seemliness, severity,
   shapelessness, simpleheartedness, simplemindedness, simpleness,
   simplicity, sincerity, single-heartedness, single-mindedness,
   singleness, singleness of heart, smallness, smoothness, soberness,
   sociability, spareness, starkness, straightforwardness, structure,
   submissiveness, talkativeness, tangibility, taste, tastefulness,
   teachableness, terseness, transparency, transpicuity, trustfulness,
   uglification, uglifying, ugliness, unadorned style, unadornedness,
   unadulteration, unaestheticness, unaffectedness, unambiguousness,
   unattractiveness, unbeautifulness, unceremoniousness, uncomeliness,
   unconstrainedness, unconstraint, unconventionality,
   unembellishedness, unequivocalness, ungainliness, ungracefulness,
   unguardedness, unhandsomeness, unidealism, uniformity,
   unimaginativeness, unimportance, unity, unmistakableness,
   unmixedness, unpleasingness, unpoeticalness, unpretentiousness,
   unprettiness, unquestionability, unrepression, unreserve,
   unreservedness, unrestraint, unrestriction, unreticence,
   unromanticism, unsecretiveness, unshapeliness, unsightliness,
   unsophisticatedness, unsophistication, unsuppression,
   unsuspiciousness, untaciturnity, unwariness, vapidity, vernacular,
   visibility, visibility unlimited, visibility zero