It was different because it remained independent during the colonial era when the rest of South East Asia was colonized. It modernized itself and worked with both British and French colonies but remained independent and chose who they wanted to work with. The territory of Siam is what is nowadays known as Thailand, so you can see that Thailand managed to develop itself and its culture independently of the colonizers who ran rampant among the South East Asian region and which caused many wars in recent times.
Yes, the Constitution resolved the weaknesses in the Articles of Confederation. It gave more power to the central government, for Shay's Rebellion helped them realize they (the govt.) needed more power. It also included the Bill of Rights, letting the citizens know what their rights were.
