Slavery had been ebbing in the western world since the Age of Enlightenment, and by the time of the American Revolution it was dying. 1771 the courts of England made a ruling re-affirming a body of case law since the 14th century that "the spirit of liberty is so deeply implicated in our constitution and rooted in our very soil that a slave or negro, the moment he lands in England, falls under the protection of the laws and in regard to all natural rights becomes eo instanti a free man" (English Common Law, Commentaries, Blackstone, 1765). The UK tolerated slavery in the colonies, but it was never afforded legal standing on the mainland. Generally there was very little slavery in Europe at that time.

Sure, go back to the Roman Empire or the Vikings and slavery was common.

A fairly significant difference of slavery in US at that time, vs slavery earlier in history or in other parts of the world, was the dehumanization of slaves. The enlightenment brought focus on humanity and rights of individuals. The U.S. constitution and democracy is a key part of that movement. So how does one then justify slavery? One chooses to justify it by saying slaves are not human - a terrible, terrible cruelty and the scars of this mindset can still be observed today.

Slavery was a key reason for the US becoming a dominant economic power! At the time of the US revolution slaves made up 20% of the US population, and a higher percentage of the economic powerhouse that was the South (e.g. in SC slaves were 40% of the population). Their proportion of the labor market was naturally higher.

40-60% of US income from export during this time came from export of cotton alone, which was nigh on solely produced by slaves. Second most important export product was tobacco - similarly produced.

The exceedingly profitable manufacture, trade and export of these products gave us the leg up that enabled us to take a leading role in the industrial revolution.

For further reading: http://m.huffpost.com/us/entry/6036840