A good soldier is like a good gun. He doesn't ask questions about where he is pointed, he just moves in that direction. A leader, by contrast, is all about asking questions - ensuring that the gun is pointed in the right direction and extremely hesitant to ever pull the trigger. You don't want a soldier who debates for months about following orders. You do want a leader who knows when to debate for months about issuing orders.
McCain's criticism of Obama's opposition to the war shows that he's not ready to be a leader. If Obama were running for general, it would be fair to criticize him for questioning our need to go to war in the first place. He is not. Obama is running for president. As such, he should be completely willing to question any use of our soldiers.
McCain, by contrast, has not seemed to show himself able to question the war. If he were still in the military, this would be admirable (I guess). Given that he's asking us to make him president, this just makes him scary. He might be a good soldier. There is nothing about his stance on the war that suggests that he'd be a good president.