The Collaborative International Dictionary of English v.0.48:

Interestingness \In"ter*est*ing*ness\, n.
   The condition or quality of being interesting. --A. Smith.
   [1913 Webster]

WordNet (r) 3.0 (2006):

interestingness
    n 1: the power of attracting or holding one's attention (because
         it is unusual or exciting etc.); "they said nothing of
         great interest"; "primary colors can add interest to a
         room" [syn: interest, interestingness] [ant:
         uninterestingness]