The existing slaves at the time would have seamlessly became citizens, and technology or higher wages would have enabled the South to maintain their cotton industry.
Of course, we probably wouldn't have icons like Michael Jordan or Muhammed Ali either...
It would have prevented the passage of the US Constitution. Most of the states in 1787 were still slave states. Had the States separated then, the British would have probably reabsorbed the United States. They kicked our A$$es in the war of 1812, and would have been even more able to defeat a non unified collection of states.
Should have picked our own damn cotton.