WordNet (r) 3.0 (2006):

unappreciative
    adj 1: not feeling or expressing gratitude; "unappreciative of
           nature's bounty"

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "unappreciative":
   against, con, disappointed, disapprobatory, disapproving,
   discontented, disenchanted, disgruntled, disillusioned, displeased,
   dissatisfied, dissenting, indignant, low, opposed, opposing, poor,
   thankless, turned-off, unapproving, uncomplimentary, unfavorable,
   ungrateful, unhappy, unmindful, unthankful