niceness has 2 distinct synonyms spread across 2 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is nastiness.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the quality of nice
the quality of being difficult to detect or analyze
“you had to admire the subtlety of the distinctions he drew”
Synonyms, antonyms and the broader and narrower relations for niceness come from Princeton University’s WordNet. Full sources and licences.