WordNet (r) 3.0 (2006):

mindlessness
    n 1: total lack of meaning or ideas [syn: inanity,
         senselessness, mindlessness, vacuity,
         pointlessness]
    2: the trait of acting rashly and without prudence [syn:
       heedlessness, mindlessness, rashness]

Moby Thesaurus II by Grady Ward, 1.0:

137 Moby Thesaurus words for "mindlessness":
   Urdummheit, accidia, acedia, acuteness, animality, apathy,
   asininity, ataraxia, ataraxy, atrocity, barbarity, battiness,
   blindness, bloodlust, brainlessness, brutality, buffoonery,
   carelessness, casualness, clownishness, crackpottedness,
   crankiness, craziness, daffiness, deafness, desipience,
   destructiveness, detachment, disinterest, dispassion, disregard,
   disregardfulness, easygoingness, eccentricity, extremity, fatuity,
   fatuousness, ferociousness, fierceness, folly, foolery,
   foolheadedness, foolishness, force, frivolity, frivolousness,
   furiousness, giddiness, goofiness, harshness, heedlessness, idiocy,
   ignorance, imbecility, impetuosity, inanity, inattention,
   incapacity, inclemency, incognizance, incomprehension, incuriosity,
   indiscrimination, ineptitude, inexcitability, inhumanity, insanity,
   insensibility, insouciance, intensity, irrationality,
   lack of affect, lackadaisicalness, lackbrainedness, lackwittedness,
   listlessness, low IQ, lunacy, madness, malignity, mental weakness,
   mercilessness, murderousness, negligence, niaiserie, nonchalance,
   nonrealization, nonrecognition, nugacity, nuttiness, pitilessness,
   pococurantism, primal stupidity, queerness, reasonlessness,
   recklessness, regardlessness, rigor, roughness, sappiness,
   savagery, screwiness, senselessness, severity, sharpness,
   silliness, slackmindedness, slackwittedness, sloth, stupidity,
   terrorism, thoughtlessness, triflingness, triviality,
   unanxiousness, unawareness, unconcern, unconsciousness,
   ungentleness, unintellectuality, unintelligence, unmindfulness,
   unsolicitousness, unwisdom, unwiseness, unwittingness, vandalism,
   vehemence, venom, viciousness, violence, virulence, wackiness,
   weirdness, witlessness, zaniness, zanyism