WordNet (r) 3.0 (2006):

unimpressive
    adj 1: not capable of impressing [ant: impressive]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "unimpressive":
   back-burner, dinky, dispensable, immaterial, inappreciable,
   inconsequential, inconsiderable, inessential, inferior,
   insignificant, irrelevant, little, minor, minute, negligible,
   nonessential, not vital, petit, small, technical, unessential,
   unnoteworthy