The body language in the string quartet added an extra element to the music being played.  Because of these movements, the audience could actually feel what the performers were feeling while playing the piece.

The movements in the violinists was most apparent.  They swayed with each change in the music, and at some strong points began stomping their feet and moving in all directions.  It was almost as if they were dancing on stage.

The cellist also made some stomping movements but was much more subtle about them (perhaps because of the size of his instrument).  The woman playing the viola did not move much, except for slight head movements during certain points in the performance.

These movements made me understand what the music was saying.  It was almost as if the performers were speaking to the audience through their music.  It helped make the music a part of them instead of something they just created.