Slavery is historically a very diverse institution.
Under the Ottoman Empire, most important political and military offices were held by slaves of the Sultan. This was VERY common in the Muslim world. In fact, one of the Delhi Muslim sultanates was called the Slave Dynasty.
http://en.wikipedia.org/wiki/Mamluk_Sultanate_(Delhi)
In the early Roman Empire, freedmen and slaves formed the staff of the Emperor and essentially ran everything. Freedmen were still clients, not really free in the sense we would use the term.
Under the Persians and other Oriental Empires, all, even the nobles, referred to themselves as slaves of the King, and not just in a figurative sense.
For most of human history, there was a spectrum of human status and legal position. Various categories of slaves, serfs, freedmen, various levels of nobles, all the way up to the King. The difference between gradations on this spectrum were slight and therefore not that important.
Along comes the Declaration of Independence. All men are created equal, except of course for the slaves. Well then they must not really be men, creating probably a harsher separating line between slavery and freedom than had ever existed before.
This can be seen by the situation in Latin America, where nobody ever proclaimed all men were equal. Slavery declined and eventually disappeared, for the most part, more easily. Perhaps largely because there was less difference between a slave and a low-status “free” person in these countries, where class still reigned supreme.
Korea used to have a large caste of state slaves