The Collaborative International Dictionary of English v.0.48:

Featureless \Fea"ture*less\ (?; 135), a.
   Having no distinct or distinctive features.
   [1913 Webster]

WordNet (r) 3.0 (2006):

featureless
    adj 1: lacking distinguishing characteristics or features; "the
           featureless landscape of the steppe"

Moby Thesaurus II by Grady Ward, 1.0:

110 Moby Thesaurus words for "featureless":
   abstract, amorphic, amorphous, anarchic, articulated, baggy, bare,
   barren, bland, blank, bleached, blobby, blurred, blurry, broad,
   catenated, ceaseless, chaotic, characterless, clear, collective,
   concatenated, confused, connected, constant, continual, continued,
   continuing, continuous, cyclical, devoid, direct, disorderly,
   empty, endless, formless, fuzzy, gapless, general, generalized,
   generic, hazy, hollow, immediate, inane, incessant, inchoate,
   indecisive, indefinite, indeterminate, inform, insipid,
   interminable, joined, jointless, kaleidoscopic, linked, lumpen,
   misty, monotonous, nebulous, neutral, never-ending, nondescript,
   nonspecific, nonstop, null, null and void, obscure, orderless,
   perennial, periodic, recurrent, repetitive, round-the-clock,
   running, seamless, serried, shapeless, smooth, stable, steady,
   straight, twenty-four-hour, unbroken, unceasing, uncharacterized,
   unclear, undefined, undifferentiated, unending, uniform,
   unintermitted, unintermittent, unintermitting, uninterrupted,
   unordered, unorganized, unrelieved, unremitting, unspecified,
   unstopped, vacant, vacuous, vague, void, white, wide,
   with nothing inside, without content