The Collaborative International Dictionary of English v.0.48:

Tasteless \Taste"less\, a.
   1. Having no taste; insipid; flat; as, tasteless fruit.
      [1913 Webster]

   2. Destitute of the sense of taste; or of good taste; as, a
      tasteless age. --Orrery.
      [1913 Webster]

   3. Not in accordance with good taste; as, a tasteless
      arrangement of drapery; a tasteless remark.
      [1913 Webster] -- Taste"less*ly, adv. --
      Taste"less*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tasteless
    adj 1: lacking flavor [ant: tasty]
    2: lacking aesthetic or social taste [ant: tasteful]

Moby Thesaurus II by Grady Ward, 1.0:

150 Moby Thesaurus words for "tasteless":
   Doric, arid, barbarian, barbaric, barbarous, barren, base, blah,
   bland, blank, bloodless, boorish, cacophonous, changeable,
   characterless, cheap, chintzy, clumsy, coarse, cold, colorless,
   crude, dead, dilute, diluted, dismal, distasteful, doggerel,
   draggy, drearisome, dreary, dry, dryasdust, dull, dusty,
   dysphemistic, effete, elephantine, empty, etiolated, fade, flashy,
   flat, flavorless, garish, gauche, gaudy, graceless, gross, gruelly,
   halfhearted, harsh, heavy, ho-hum, hollow, ill-flavored, improper,
   impure, in bad taste, inane, inappropriate, inconcinnate,
   inconcinnous, incorrect, indecent, indecisive, indecorous,
   indelicate, indifferent, inelegant, inexcitable, infelicitous,
   infirm of purpose, infirm of will, insipid, irresolute, jejune,
   leaden, lifeless, loud, low, low-spirited, maladroit, meretricious,
   mild, milk-and-water, milky, mushy, neutral, offensive, outlandish,
   pale, pallid, pappy, pedestrian, plodding, pointless, poky,
   ponderous, pulpy, rude, sapless, savorless, slow, solemn,
   spiceless, spiritless, stale, sterile, stiff, stodgy, stuffy,
   superficial, tawdry, tedious, thin, unaesthetic, unappetizing,
   unbecoming, unbeseeming, uncourtly, uncouth, uncultivated,
   uncultured, undignified, uneuphonious, unfelicitous, unfitting,
   unflavored, ungenteel, ungraceful, uninteresting, unlively,
   unpolished, unrefined, unsavory, unseemly, unsuitable, untasteful,
   vapid, vulgar, washy, watered, watered-down, watery, weak, wild,
   wishy-washy, wooden, wrong