The question of where Jesus stood on the question of slavery is interesting.* It is true that nowhere is it directly addressed in the New Testament. But that doesn't mean that Jesus or his disciples or immediate successors and worshippers were in favor of it. Silence does not necessarily imply endorsement. I suspect that, for them, it simply wasn't an important issue as they themselves were neither slaves nor slave -owners.
I've been doing some reading recently on the Essene sect of Jesus' time in connection with some research on the Dead Sea Scrolls. According to Philo of Alexandria, a contemporary of Jesus, as well as Flavius Josephus, the Jewish soldier and historian who was also a contemporary, the Essenes were opposed to slavery and found it an unnatural institution. Now, a lot of what Jesus is quoted as saying dovetails quite nicely with the Essenic philosophy in general. (There's at least one Christian theologian/historian who maintains that virtually all the early Jewish converts to Christianity came from the ranks of the Essenes rather than the Pharisees or Saducees.) Is it too far-fetched to suppose that Jesus, too, may have been opposed to slavery but did not think it a significant enough problem, from a spiritual viewpoint, to be specifically addressed in any of the well-known sermons?
*I wrote this initially as a reply to Thomas' post but he had deleted his post before I finished typing my reply.