It has alot to do with the religious ethics they have in the countries of the Middle East. For them it is not just a religion it is a way of life and for some Saddam Hussein is their prophet.
The war on Afghanistan was different- the country was almost defenceless apart from a few hundred thousand Al-Qaida fighters, but Iraq has nuclear weapons.
Its scary. The Middle East is very volatile and needs to be sorted, but war on Iraq I do not think is the way.