WordNet (r) 3.0 (2006):

letdown
    n 1: a feeling of dissatisfaction that results when your
         expectations are not realized; "his hopes were so high he
         was doomed to disappointment" [syn: disappointment,
         letdown]

Moby Thesaurus II by Grady Ward, 1.0:

105 Moby Thesaurus words for "letdown":
   abasement, abatement, allayment, alleviation, arrest, assuagement,
   bafflement, balk, betrayed hope, blasted expectation,
   blighted hope, blow, blunting, buffet, calming, check, comedown,
   cruel disappointment, damping, dash, dashed hope, deadening,
   debasement, deceleration, defeat, deflation, delay, demulsion,
   descent, detention, diminution, disappointment, discomfiture,
   disgrace, disillusionment, dissatisfaction, drag, dulcification,
   dulling, dump, ease-off, ease-up, easing, embarrassment, failure,
   fallen countenance, falling-off, fiasco, fizzle, flagging, foiling,
   forlorn hope, frustration, hangdog look, holdup, hope deferred,
   humbled pride, humiliation, hushing, lag, leniency, lessening,
   letup, lightening, loosening, lulling, minus acceleration, mirage,
   mitigation, modulation, mollification, mortification, obstruction,
   pacification, palliation, put-down, quietening, quieting,
   reduction, relaxation, remission, retardation, retardment,
   self-abasement, self-abnegation, self-diminishment, setback,
   setdown, shame, shamefacedness, shamefastness, slack-up,
   slackening, slowdown, slowing, slowing down, slowup, softening,
   soothing, sore disappointment, subduement, tantalization, tease,
   tempering, tranquilization