Tastefulness is the noun: the quality of being tasteful. It names the thing itself — the good judgement, the restraint — rather than describing an object. 'The tastefulness of the design', 'a certain tastefulness in everything she wore'. Here is the honest part, and it matters more than the meaning: this word is rare. It is correct English, it appears in dictionaries, but people almost always find another way to say it. They say 'good taste' instead — 'the good taste of the design', 'she has wonderful taste'. Written down, tastefulness feels heavy and slightly academic, the kind of word you meet in an art review. So learn it so that you recognise it when you read it, but when you speak or write, reach for 'good taste'. That is not a rule, just what will make you sound like everyone else. Notice the pattern behind it, though, because that pattern is very useful: -ness takes a describing word and makes a noun for the quality. Kind → kindness, dark → darkness, careful → carefulness, tasteful → tastefulness. Once you see it you can read hundreds of words you have never met.

品味、雅緻（此字少用，口語多說 good taste）