As I understand it, "War" triggers an array of legal and treaty issues that restrict and complicate our options. Being that this is tied to the war on terrorism, it may spill over there. What pops into mind first is the treatment of POWs: No interrogations, regular mail delivery, furnishment of "scientific instruments", religious services etc… I know there’s more to it than just the POW issue, but that’s all I recall.
Now of course the inevitable call will come, "why are we in these treaties anyway", and "why don’t we just nullify them and call it war anyway". There are political answers to that involving world PR, domestic politics, timing, and the difficulty of reestablishing them next time if we are at war with a developed "civilized" nation, but that’s another long explanation. So rather than get bogged down in redefining a formal fraimwork for a new kind of war (as the publics memory of 9/11 fades), we’ve chosen an authorization method that is without a doubt constant with the sprit of the Constitution, has been upheld by courts and satisfies all but a tiny minority that are represented here.