George Washington's most important lasting impact came from the fact that he walked away from the Presidency after two terms in office. This made it possible for the United States to be what it is today.
In many new countries, popular leaders like Washington stay on as long as possible. They make themselves into dictators and suppress political opposition. A good example of this is Robert Mugabe in Zimbabwe today. Washington could easily have become like this because he was so popular.
Instead, Washington stepped down from office and set a precedent. He established the idea that the US really would be a nation of laws and not a country where leaders would hold on to power until they were forced to step down. Because of this, the US has enjoyed a stable and democratic government ever since.