Due to his status as a revolutionary general, the first American President, and the father of the nation, George Washington earned a truly stellar reputation that endured for centuries. Though many other figures from early American history had more contested legacies—Jefferson immediately springs to mind—Washington's reputation remained secure, almost completely beyond all criticism.
Over time, however, even Washington became a much more controversial figure. To a large extent, this was due to his relationship with the institution of slavery. Critics pointed out that, whatever Washington's achievements as general and President may have been, he was still very much a man of his time and class in that he owned a substantial number of slaves.
Some historians pointed out that Washington's treatment of his slaves, though perhaps not as bad as that of other slave-owners, was nonetheless often quite harsh. Washington personally approved of the physical punishment of slaves for minor transgressions. In one notorious case, he approved of the beating of a seamstress as punishment for arguing with the farm manager and refusing to work.
Such historical facts paint Washington in a different, much less heroic light than traditional representations such as Emanuel Leutze's famous painting of Washington crossing the Delaware River. This doesn't mean, of course, that Washington's service to the nation and his major contributions to public life should be ignored. But it does mean that, in properly evaluating the reputation of the first President, one needs to adopt a much more critical, less reverential approach.