This scene is iconic with the Doctor Who series. I remember first seeing this at the beginning of The Five Doctors anniversary episode. That episode was the one that led me to seek out the other Doctor's episodes and watch them.
Its sad as the Doctor says farewell to what maybe the last of his family. Susan is the only real blood family member he ever had on the show. Although I think Carole Ann Ford played the character a little to weird for my tastes it is still sad to see her go. She was starting to grow on me and I would really love to see her story revisited in the new series. We never do find out what happens to her do we.