The Collaborative International Dictionary of English v.0.48:

Obtrusive \Ob*tru"sive\, a.
   Disposed to obtrude; inclined to intrude or thrust one's self
   or one's opinions upon others, or to enter uninvited;
   forward; pushing; intrusive. -- Ob*tru"sive*ly, adv. --
   Ob*tru"sive*ness, n.
   [1913 Webster]

         Not obvious, not obtrusive, but retired. --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

obtrusiveness
    n 1: an unwelcome conspicuousness [ant: unobtrusiveness]