His poetry is known for its emotional expressiveness.

expressiveness 是由形容词 expressive（富有表现力的）加上名词后缀 -ness 构成的名词。expressive 来自动词 express（表达），加上形容词后缀 -ive 构成。expressiveness 指"表现力、表达能力"，在这里特指诗歌中情感的表达力度。