Expressiveness is to express, to communicate, to say something, to share an idea or a thought. I think that expression is a basic caracteristic in design and everything in design is expressiveness.
Some years ago I did a group work in my bachelor in Spain that is about this. We talked about expressiveness in relation to the Spanish written special characters, like swung dash. In this work we made the swung dash communicate anger or surprise, related with the face gesture. Personally, the importance of this work was not the execution but in the discovery for me of the direct relation between language, expression, the way of writting something and the shape of the typo or other characters, very important even if they are “specials”!
Expression is communication in every forms, from music or cinema to a shout or a gesture. To express is to share information or emotions.