so far i know 1. Industry: The northern states had industrialized, creating factories, and had a tremendous shipping and fishing industry, and thus did not have a need to keep slaves. Mind you, there WERE slaves in the North, but not nearly as many as in the South. The South, on the other hand, relied on plantation crops such as cotton and tobacco, and thus needed large numbers of slaves to farm the land. Without this virtually free labor, they would not have earned the profits they did on these crops.
2. The North, while it had slaves, did not rely on them because they had industrialized, with far fewer farms. The South, with massive plantations and farms all over the place, had a labor problem, and continued to rely on slaves until the end of the Civil War.
Those are probably the the two biggest ones (slaves and industry).