Its already been mentioned, but all the major revelations of the Original Trilogy are spoiled in the prequels.
The big one :Darth Vader is Luke's father. The scene that had some people gasping in theaters and debating it until Return of the Jedi came out would carry little weight if you watch from I to VI in that order.
Heck. You would even know Luke and Leia are siblings before they kiss. X)