Source : Webster's Revised Unabridged Dictionary (1913)

Niceness \Nice"ness\, n.
   Quality or state of being nice.

Source : WordNet®

niceness
     n 1: a courteous manner that respects accepted social usage [syn:
           {politeness}] [ant: {impoliteness}]
     2: the quality of nice [ant: {nastiness}]
     3: the quality of being difficult to detect or analyze; "you
        had to admire the subtlety of the distinctions he drew"
        [syn: {subtlety}]