Yes, numbers and supplies were the biggest advantage. But probably next on the list was general marksmanship. The United States was still very rural at this time, so many soldiers had previous experience with firearms and additionally marksmanship was an intense focus during training. For more see http://en.wikipedia.org/wiki/Battle_of_Belleau_Wood

In terms of weaknesses, a big one would be large unit command and control. The professional army before World War I, was still very much a frontier army working with small detachments. It had very few officers with experience to command large formations.