The Ariticles of Confederation created a federal government that was too weak to be effective. The government was made of a legislative body that was unicameral, that is a single house legislature. The Articles of Confederation also made another important mistake by not allowing the government to collect taxes. In short, the states held all the power and would not allow the federal government to have any real power.
The Constitutional Convention in Philadelphia was originally called to revise the Aritices of Confederation. The debate however was focused on the effectiveness of the existing government. The consensus was that the government was not working and was not truely uniting the 13 states like it was originally designed to do.
The United States constitution was designed to create a strong federal government capable of doing six things. These goals are spelled out in the Preamble.
The first test of the strength of the new government came at the Whiskey Rebellion. Under the Aritcles this would have never happened, because the trouble centered around a federal tax on whiskey. The people of western Pennsylvania didn't think they should have to pay a federal tax because they just basically disagreed with the tax. The state government refused to enforce the federal government's tax. The newly created office of President was forced into a position to enforce a law that the Congress had created. The ultimate test of the strength of the constitution. The actions of the president did prove successful in putting down the rebellion.