I think "Old Pete" had caught a lot of flack for a) assisting in the Reconstruction and b) not being T.J. Jackson.

Strategically, Longstreet was correct.

When McClellan found just enough advantages to make him actually fight at Sharpsburg -- and he preferred chess only when he could remove your rooks before playing -- the AotP fought Lee's troops to a very bloody draw, which the Union labeled a victory. While undefeated tactically, Lee's boys headed home because they simply didn't have enough resources left to continue an offensive campaign. I suspect that Longstreet knew then that the South could not win some great victory on the battlefield or capture Philadelphia and end the war with a new nation. I believe that he grasped that their only remaining hope was to assume the tactical defense whenever and wherever possible so as to inflict grossly disproportionate casualties on the North and wear down their resolve. He wanted the rest of the South's war to be a repeat of Marye's Heights....not Gettysburg or even Chancellorsville. The former ended in defeat and the latter in victory, but the South could not afford the casualties it took in either.

He had grasped that with the new killing depth presented by the rifled musket -- a beaten zone of 300-400 yards as opposed to 100-150 -- that attacks against an enemy with any kind of reasonable shelter were going to produce ghastly casualties. When Napoleon's troops closed the enemy, they would come under meaningful fire only 100 yards away from their enemy. Unless that enemy was a machine-like regiment capable of 5 rounds a minute, the charging troops faced only one or two volleys before it came to the steel. In the ACW, a typical unit could get off about 3 rounds a minute while attacking troops could cover roughly 100-150 yards a minute and had 2 or more minutes to engage the closing enemy. Until the development of LMGs and Light mortars, the defense had a crucial advantage.

I think Pete wanted to fight with what would come to be known as the Von Moltke approach (not that he was the first to use it). Move rapidly to a good defensive position that for logistic or political reasons your enemy cannot let you keep. Then let them pay the blood price. This combines the strategic offensive (choosing where and when) with the tactical defensive in a best of both worlds.

Longstreet never did have Jackson's knack for making infantry go ridiculous distances quickly though. I don't know of many commanders who did aside from the young Bonaparte. Longstreet was pretty good at getting troops moving, but it was a slow era. Move 'em too fast and too few of them would arrive able to fight. Move 'em a bit slow and you take all morning to get going after Little Round Top. No really good answers.

As to Gettysburg, I have long thought that the slowness at Culp's Hill on the Union Right may have been every bit as problematic -- or more -- than was the delayed effort to go around the Union left. Buford won the battle on day one by giving the Union the better ground and by getting Lee's blood up enough for him to want to take it anyway.