The Collaborative International Dictionary of English v.0.48:

Mildness \Mild"ness\, n.
   The quality or state of being mild; as, mildness of temper;
   the mildness of the winter.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mildness
    n 1: good weather with comfortable temperatures [syn:
         mildness, clemency]
    2: acting in a manner that is gentle and mild and even-tempered;
       "his fingers have learned gentleness"; "suddenly her gigantic
       power melted into softness for the baby"; "even in the pulpit
       there are moments when mildness of manner is not enough"
       [syn: gentleness, softness, mildness]
    3: mercifulness as a consequence of being lenient or tolerant
       [syn: lenience, leniency, mildness, lenity]

Moby Thesaurus II by Grady Ward, 1.0:

103 Moby Thesaurus words for "mildness":
   abnegation, abstinence, acceptance, affability, amiability,
   bonhomie, calm, calmness, clemency, clementness, compassion,
   conservatism, constraint, continence, control, cool, cordiality,
   deadness, dispassion, dovelikeness, easiness, easygoingness,
   evenness, flatness, flavorlessness, forbearance, forbearing,
   geniality, gentleness, golden mean, good disposition, good humor,
   good nature, good temper, goodnaturedness, goodtemperedness,
   happy medium, humaneness, humanity, humility, impartiality,
   inanity, insipidity, insipidness, jejuneness, jejunity,
   judiciousness, juste-milieu, lamblikeness, laxness, lenience,
   leniency, lenientness, lenity, lifelessness, meden agan, meekness,
   mercifulness, mercy, middle way, moderateness, moderation,
   moderationism, neutrality, nonviolence, nothing in excess,
   pacifism, patience, peaceableness, pity, prudence, quietness,
   repose, restraint, saplessness, savorlessness, self-abnegation,
   self-control, self-denial, self-restraint, serenity, sobriety,
   softness, stability, staleness, steadiness, sweetness, tameness,
   tastelessness, temperance, temperateness, tenderness, thinness,
   tolerance, tranquillity, unexcessiveness, unextravagance,
   unextremeness, unsavoriness, vapidity, via media, weakness,
   wishy-washiness