WordNet (r) 3.0 (2006):

unenthusiastic
    adj 1: not enthusiastic; lacking excitement or ardor; "an
           unenthusiastic performance by the orchestra";
           "unenthusiastic applause" [ant: enthusiastic]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "unenthusiastic":
   apathetic, backward, balking, balky, blase, cold, cool, desireless,
   dilatory, grudging, halfhearted, inappetent, indifferent, laggard,
   loath, loveless, lukewarm, nirvanic, passionless, perfunctory,
   reluctant, renitent, restive, slow, slow to, un-come-at-able,
   unambitious, unapproachable, unaspiring, unattainable, unattracted,
   undesirous, uneager, unexcited, ungetatable, unimpressed,
   uninterested, unobtainable, unreachable, unresponsive, unzealous