First of all, though the resolution does reference the "threat" posed by Iraq, it also talks specifically about dealing with those who were involved with the 9/11 attacks. Iraq wasn't involved.
Secondly, the resolution calls for the U.S. to "enforce all relevant United Nations Security Council resolutions regarding Iraq." I don't know about you, but I have a problem with a multi-national coalition enforcing the will of a global, socialist organization. Under any other circumstances, we would call that world government.
Thirdly, Congress authorized the president to use the "Armed Forces of the United States as he determines to be necessary and appropriate." This isn't anything even remotely resembling a declaration of war. Either we are in a state of war, or we aren't. This resolution merely passes the buck to the president, allowing one man to determine whether or not we are at war. If this were any other country, that kind of power in the hands of one man would be called tyranny.
Why are so many people willing to destroy our constitutional republic in order to save it?