The most important precedent that George Washington set was to leave office peacefully after his terms in office. One of the major problems that many other young countries have had is that their first leader tries to become a king and to hold on to power indefinitely. This sets a precedent where every leader tries to do the same. By contrast, Washington left peacefully, setting the precedent that power should be transferred peacefully and democratically. This is still done today.
A second precedent that has only been broken once, and is now in the Constitution, is that Washington left after two terms in office. This is related to the first precedent because it set a precedent for one president leaving so he (or someday she) could not become president for life or anything like that. Only FDR has ever been elected to more than two terms and the Constitution now sets Washington's precedent as law.