Actually, Brees doesn't really throw half his passes that hard. What differentiates him from other gun-slingers is that he finds the open reciever and he puts a touch on the ball which many qbs don't have.
It's not his arm--physically, he's not that imposing or gifted. The guy is very intelligent and makes very smart decisions.
His ability to read defenses and make good decisions makes him great. I liken him to Joe Montana.