The Third Corp ( Hill's) was very active on the first day, they lead the way before Ewell came into the fight on his left. Then on the personal side, AP Hill was once again hit with his sickness which seemed to follow him during this time so he was not all together on top of his game.
The Third Corp had taken its share of losses and being July 1st was very hot and muggy, his Corp wasn't all that fresh to begin the 2nd with. Add to the fact this was the first major battle which Hill commanded a Corp from the beginning of a battle. The soldiers had been marching days before hand before reaching Gettysburg. Lee was still wondering if he really needed to enter into a major engagement since his army was spread out and had not heard anything from JEB Stuart as to the location of the Union army.
Gettysburg to me and I assume many other Civil War students look at this battle with as many "what if's" as any other battle in the war.