Within tango you often hear the word connected. I think one of the important aspects explaining the difference is the source of the movement, where the movement starts.
Both partners are initiating their own movements when they understand that the other person is reacting on music. You can start a serie of steps when you see/understand that the other person is going to start on a certain beat in music.
In connected dance you take a step when you in your body feel what kind of step the other person is doing. The information goes both ways and often simultaneously.
I think in most of the couple dances the dancers are growing towards the connected dance, but for the Argentine tango it is the starting point.