Population: the North had 22 million people at the start of the civil war, while the south had 9 million, less than half. This proved crucial because it allowed the North to field larger armies over a longer period of time (especially when soldiers began to die in large numbers).
Industry: the North was industrializing, while the South was essentially agricultural. This meant that the North had better supply lines, more railways, and more factories.
Leadership: most historians agree that Abraham Lincoln was a far more effective leader than Jefferson Davis.
The South had many disadvantages and most historians agree that winning the war for the Confederacy was practically impossible, however one advtange the South had was:
Territory: the Confederacy was vast, rural, the climate was hot and humid, and tropical diseases were common. The northern soldiers had it harder to adapt to this conditions, and the North had to invade the vast territory while the Confederacy only had to defend.
