The Collaborative International Dictionary of English v.0.48:

Pensiveness \Pen"sive*ness\, n.
   The state of being pensive; serious thoughtfulness;
   seriousness. --Hooker.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pensiveness
    n 1: persistent morbid meditation on a problem [syn:
         pensiveness, brooding]
    2: deep serious thoughtfulness [syn: pensiveness,
       meditativeness, contemplativeness]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "pensiveness":
   absorption, abstraction, brown study, castle-building, close study,
   concentration, contemplativeness, daydreaming, deep thought,
   dreamery, dreamfulness, dreaminess, dreaming, dreamlikeness,
   engrossment, fantasying, gentle melancholy, melancholia,
   melancholiness, melancholy, musefulness, musing, pipe-dreaming,
   preoccupation, profound thought, reflectiveness, reverie,
   romantic melancholy, speculativeness, study, thoughtfulness,
   tristfulness, wistfulness