Well, Thande's idea about Muslim slaves is certainly interesting, but I think it might be harder to get a steady supply from the Ottomans et al. Certainly, it would alter the currents of trade, since the Trianlge Trade would have to flow through the Med or around the Cape of Good Hope. It would seem however that theories of racial inferiority associated with slavery would be vastly different: if a Muslim slave converted to Christianity would he be freed?
If so, then slavery will not make sense for the early colonial powers like the Portugese and the Spanish, who would also want to spread Christianity. However, if the Africans don't practice slavery, then the Spanish and Portugese may not try to enslave native American populations and instead reduce them to something like serfdom.
All in all, I'd imagine that indentured labor takes the place of slavery in terms of serving the need to import manual labor to the New World. The question here is does indentured labor evolve into full fledged debt-slavery? I imagine it largely depends on the economic returns involved.
Consider sugar cane planatations, which thrived OTL due to their ability to work their slave labor to death. If it's cheaper to work a slave to death than to preserve the capital invested in his or her purchase, presumably it is even better to do so with indentured laborers. However, indentured servitude is nominally voluntary and who would volunteer to work to death? If they had to depend on indentured servitude for labor, I don't think sugar plantations could have maintained the kind of profit margins they enjoyed OTL.
Cash crops whose cultivation do not require such poor conditions may be another matter all together. Working a tobacco or cotton field in Virginia or South Carolina is presumably little different from working a field in Europe (except for the differing diseases involved). And of course there's the incentive that the laborer upon finishing his term of service would have learned a more profitable skill (farming a cash crop).
However, there's still a problem: can you take out debt on the labor owed you by an indentured servant? I think the answer is no. The whole reason the person is obligated to you is that he or she in debt to you. From an economic point of view, the price of the servant should capture the future benefits the master would accrue due to the servant's labor. And of course the servant is not property; after some term of service, he or she is free and hence of no inherent economic value to the master. This drastically reduces that ability to borrow against the value of the forced labor pool, which was a characteristic practice among slaveholders in plantation economies. Without this access to debt afforded by holding the labor, the mechanics of a plantation economy are drastically changed. The master must focus his efforts on attaining productive value from the servant, rather than living off of borrowed money (as many slaveholders did, in the United States). Planters' income would now depends much more on their ability to make money, rather than borrow against the value of their slave. They can still borrow against the price of their land or their future crops, but they must bare the expense of the servant as such.
Now, this difference in my mind drastically changes the appeal of engaging in plantation agriculture. In any case, it changes the mindset of that culture fairly fundamentally. I see two cases, depending on the actuall degree of profitability of the altered system of indentured labor: if indentured labor is more profitable than simple agriculutre, then indentured labor continues, but the planters of the system take on much more the mindest of a robber baron than an aristocratic grandee (i.e. a manager rather than a person of leisure). If not, then the New World becomes largely a haven for dissident seeking a new life.
I think the answer must be some kind of combination of the two, depending on later circumstances and events.
I would also note that I wonder if theories of racism might persist far longer than they did OTL. Note that I would separate here the racism engendered by race slavery and that engendered by the difference between peoples of different nations (i.e. Asians and Europeans). Presumably the lack of the former would affect the tenor of the later, but the later would still exist, in the samer manner that Englishman of the 1700s had a complex hierarchy of the relative merits of different races. I suppose if this were the case, race might never truly differentiate from nationality in the sense it did (i.e. theories of Mogloid, Negroid, and Caucasian races).
