The September 8th, 2001 Awake aimed to answer this question. To say the least, this is a topic of great controversy and embarrassment for Christains.
Essentially, the Awake said that God is unequivocally opposed to oppresive forms of slavery (as if any type of slavery is not considered oppresive to the one being enslaved) and that he demonstrated his opposition to such slavery by freeing the Isrealites from the oppresive Eyptians (but of course leaving slaves under more despotic conditions to fend for themselves).
They acknowledged that God allowed the Isrealites to own slaves, but not opression of them (an example given was that the Isrealites could take women as slaves, but could only have sex with them if they made them their wives. In the "opressive" forms the women would have sex with their masters without the benefit of marriage and only for the masters sexual gratification.) If God thought all men were created equal, why would he allow slavery? (Maybe because the slaves laws weren't actually made by him
As for slavery in the times of Rome, the Awake simply states : "Slavery was part of the economic system of the Roman Empire... But does this mean that disciples of Christ were abusive slaves owners? Hardly! Regardless of what Roman law permitted, we can be sure that Christains did not mistreat those under their authority. (Tell that to an African slave who was taken to America, and more than likely had a Christain master.)
And of course, in typical JW fashion, they said that all the problems of slavery and man dominating man will be solved in the new system (I guess even "good" Isrealite- Christain slavery will be condemned by God then ).
I found the whole argument of "Good Slavery- Bad Slavery" quite unsatisfactory. They're other more benign areas that had no gray areas (e.g. extra marital sex) but with something as life altering and potentially dangerous as slavery, God gave the thumbs up. For those of you who can reconcile the idea of God condoning slavery and being good (or just anyone with a comment) please tell how you do it, because it is beyond my comprehension how most can do it.
Go therefore and baptize the people in the name of the father and of the son... what the hell, we just need to bring up the yearbook numbers!