WordNet (r) 3.0 (2006):

let down
    v 1: move something or somebody to a lower position; "take down
         the vase from the shelf" [syn: lower, take down, let
         down, get down, bring down] [ant: bring up,
         elevate, get up, lift, raise]
    2: fail to meet the hopes or expectations of; "Her boyfriend let
       her down when he did not propose marriage" [syn:
       disappoint, let down]

Moby Thesaurus II by Grady Ward, 1.0:

184 Moby Thesaurus words for "let down":
   abate, apostatize, arrest, backpedal, backwater, baffle, baffled,
   balk, balked, bamboozle, bate, bear down, beguile, betray,
   betrayed, bilk, bilked, blasted, blighted, bluff, bolt, brake,
   break away, bring low, cajole, cast down, chapfallen, cheat on,
   check, circumvent, clip the wings, conjure, couch, crestfallen,
   cross, crossed, crushed, curb, dash, dashed, debase, deceive,
   decelerate, defeat, defeat expectation, defeated, defect, deflate,
   degenerate, delay, delude, demit, depress, desert, detain,
   deteriorate, detrude, diddle, disappoint, disappointed, disgrace,
   dished, disillusion, disillusioned, disimprove, dissatisfied,
   dissatisfy, double-cross, downbear, draw rein, droop, dupe, ease,
   ease off, ease up, embarrass, fall back, foil, foiled, forestall,
   frustrate, frustrated, gammon, get around, get worse, grow worse,
   gull, haul down, hoax, hocus-pocus, hold back, hold in check,
   hold up, hornswaggle, humble, humbug, humiliate, ill done-by,
   ill-served, impede, indent, inform on, juggle, keep back, let up,
   let up on, loose, loosen, lose ground, lose momentum, lose speed,
   lower, mitigate, mock, moderate, mortify, obstruct,
   out of countenance, outmaneuver, outreach, outsmart, outwit,
   overreach, pigeon, play one false, press down, pull down, pull out,
   push down, put out, put something over, put to shame, rat, reduce,
   reef, regress, regretful, rein in, relapse, relax, remit, retard,
   retrograde, retrogress, run out on, secede, sell out, set back,
   shame, sicken, sink, slack, slack off, slack up, slacken, slake,
   slip back, slow, slow down, slow up, snow, sorely disappointed,
   soured, stay, string along, take down, take in, take in sail,
   tantalize, tease, throttle down, thrust down, thwart, thwarted,
   trick, turn in, two-time, unbend, unbrace, unlax, unstrain,
   unstring, unwind, worsen