The Collaborative International Dictionary of English v.0.48:

disappointing \disappointing\ a.
   defeating one's expectations or hopes; failing to fulfill
   one's expectations or hopes; as, a disappointing result; a
   disappointing crop yield.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

disappointing \disappointing\ n.
   the act of disappointing someone.

   Syn: disappointment, dashing hopes.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Disappoint \Dis`ap*point"\, v. t. [imp. & p. p. Disapointed;
   p. pr. & vb. n. Disappointing.] [OF. desapointier, F.
   d['e]sappointer; pref. des- (L. dis-) + apointier, F.
   appointier, to appoint. See Appoint.]
   1. To defeat of expectation or hope; to hinder from the
      attainment of that which was expected, hoped, or desired;
      to balk; as, a man is disappointed of his hopes or
      expectations, or his hopes, desires, intentions,
      expectations, or plans are disappointed; a bad season
      disappoints the farmer of his crops; a defeat disappoints
      an enemy of his spoil.
      [1913 Webster]

            I was disappointed, but very agreeably. --Macaulay.
      [1913 Webster]

   Note: Disappointed of a thing not obtained; disappointed in a
         thing obtained.
         [1913 Webster]

   2. To frustrate; to fail; to hinder of result.
      [1913 Webster]

            His retiring foe
            Shrinks from the wound, and disappoints the blow.
                                                  --Addison.

   Syn: To tantalize; fail; frustrate; balk; baffle; delude;
        foil; defeat. See Tantalize.
        [1913 Webster]

WordNet (r) 3.0 (2006):

disappointing
    adj 1: not up to expectations; "a disappointing performance from
           one who had seemed so promising" [syn: disappointing,
           dissatisfactory, unsatisfying]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "disappointing":
   disconcerting, discouraging, disheartening, displeasing,
   dissatisfactory, falling short, inadequate, incommensurate,
   inferior, insufficient, not good enough, pathetic, poor, sad,
   second best, second-rate, sorry, tantalizing, teasing,
   unfulfilling, ungratifying, unsatisfactory, unsatisfying