I think that the first answer was able to capture the foreign impression and cost of Shays' Rebellion. Along with that, I would only add that Washington and other framers feared Shays' Rebellion because of the hypocritical reality that would be imposed upon the new nation. The Framers understood that their "anti- establishment" position against England was the reason for their success. The Revolution won international support and galvanized local sustenance because it was able to show the power of freedom and the motivating force of fighting for what one thought was right. The Framers' recognition of the concern over Shays' Rebellion demonstrates how the Framers realized that gaining independence would be easier that maintaining it. The elements that help to feed revolutionary fervor are often opposite of the ones that are required to govern effectively. This became a concern for the Framers, in that Shays' Rebellion would expose the new nation to not be one that speaks for freedom and independence, but rather for suppression and control, the very same elements against which the Framers fought England. It was here that Washington and others Framers understood that the event could have the potential to "render ourselves ridiculous."
George Washington was worried about this because he knew that most people in Europe did not think that the United States would really be able to function as a country.
In those days, most thinkers were of the opinion that a country with a system of democratic government could never last. It was believed that such a system would fall apart because it lacked a real leader. Washington worried that Shays's Rebellion would show that the US's experiment in democracy was falling apart just as the Europeans thought it would.
Because of this, Washington worried that the rebellion would confirm the predictions of the Europeans, and prove that a democracy could not last.