When I lived in Spain, salads were always simply dressed with Olive Oil, Vinegar and Salt and Pepper. There was never a need for anything else. They generally use Wine or Sherry Vinegar in Spain but Balsamic works as well. The better the quality of the Oil and Vinegar used, the less you have to use and of course, the better the flavour.
This Sicilian Extra Virgin Olive Oil and 12 year old Balsamic Vinegar from the Lilliput Trading Company dress my salads these days and they are beautiful together. Only a small amount of the vinegar is needed because it really dense (1.32 density which is pretty dense in Balsamic terms) and it will overpower everything else if you use too much. The Olive Oil Cold pressed and is Single Origin so it really holds its fruity flavour.