The Collaborative International Dictionary of English v.0.48:

Thorny \Thorn"y\, a. [Compar. Thornier; superl. Thorniest.]
   [Cf. AS. [thorn]orniht.]
   1. Full of thorns or spines; rough with thorns; spiny; as, a
      thorny wood; a thorny tree; a thorny crown.
      [1913 Webster]

   2. Like a thorn or thorns; hence, figuratively, troublesome;
      vexatious; harassing; perplexing. "The thorny point of
      bare distress." --Shak.
      [1913 Webster]

            The steep and thorny way to heaven.   --Shak.
      [1913 Webster]

   Thorny rest-harrow (Bot.), rest-harrow.

   Thorny trefoil, a prickly plant of the genus Fagonia
      (Fagonia Cretica, etc.).
      [1913 Webster]

WordNet (r) 3.0 (2006):

thorny
    adj 1: bristling with perplexities; "the thorny question of
           states' rights"
    2: having or covered with protective barbs or quills or spines
       or thorns or setae etc.; "a horse with a short bristly mane";
       "bristly shrubs"; "burred fruits"; "setaceous whiskers" [syn:
       barbed, barbellate, briary, briery, bristled,
       bristly, burred, burry, prickly, setose,
       setaceous, spiny, thorny]

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "thorny":
   Herculean, abstruse, acanaceous, acanthoid, aciculate, aculeolate,
   arduous, barbed, barbellate, brambly, briery, bristly, brutal,
   complex, complicated, controversial, critical, delicate, demanding,
   difficile, difficult, exacting, formidable, hairy, hard,
   hard-earned, hard-fought, intricate, involved, jawbreaking,
   knotted, knotty, laborious, mean, muricate, nasty, nettlesome,
   no picnic, not easy, operose, painful, pricking, prickly, pricky,
   problematic, rigorous, rough, rugged, set with thorns, setaceous,
   setose, severe, spiculate, spiculose, spiked, spinous, spiny,
   steep, sticky, stinging, strenuous, tangled, thistly, ticklish,
   toilsome, tough, tricky, troublesome, uphill, vexatious, wicked,
   worrying