Oh I completely agree with you
haku but there is one thing that you are leaving out that makes the African Slave Trade stand out a little in that it was the first instance in which slaves were reduced to things, to property and not as human beings. The very concept of
race was not conceived until the emergence of the African Slave Trade; racism was used as justification to continue the practise of slavery even though it opposed Christian values (of course, those values would also be manipulated to excuse slavery). Creating an ideology that gave license to people to treat a certain group of people a certain way because they were of a lesser species had lasting ramifications. Tell me of any exmaple you mentioned above, any civilization that kept such an exclusive and permanent status of slaves that low for that long.
Slavery did exist in the past (still does) but it was not ruled on the colour of one's skin but rather on the the acquiring of slaves in war or quite often, turning your enemies into slaves, as you said. The descendents of those slaves though often assimilated to the victor's society as was the case with Gallic slaves to Rome--eventually becoming senators--and, what I'm more familiar with, the
Mamluk of Arabia--eventually becoming conquerors in their own right. That obviously was not the case with African Slaves in this particular instance*. Descendents of the African Slave Trade are very far from integrated in their respective societies, particularly in countries like the States and Brazil (I'm leaving out the UK out of ignorance) where segregation still exists with social structures offered to inhabitants are in dismal array with persistent resistence to repair them leaving most blacks perpetually in the lower echelons of society. (Admittedly the US has really improved in the past 20 years).
Institutional Racism, what is being experienced now, is a lasting and destructive legacy of slavery and is still with us today. A government cannot apologize for slavery and wash their hands clean of its legacy while still practising institutionalized racism; it comes off as being a little insincere, IMHO.
*In this instance;
History of Blacks in Britain. Interesting, Blacks were not always seen as lesser than, they too must have assimilated into Roman society for them to be trusted to defend Roman interest in Britain.