I'm assuming this question is discussing serfdom and the peasantry. The other Educators have already discussed some substantial differences: serfdom, as brutal as it may have been, was never couched in racial terms. In addition, while tied to the land, serfs were never understood as the personal property of their owners the way slaves were.
What no one has spoken on so far is the role of religion as it existed in both the medieval European and the American context. In this case, we could note that both serfs and slaves had a very rich and powerful spiritual life, typically grounded in Christianity. Indeed, Christianity as a religion tends to be very powerful among the oppressed (often due to oppressors requiring them to convert to Christianity), but the medieval church had far greater coercive power as an institution than any slave churches would have wielded, and it did use that power to intervene for the social well-being of peasants. This is most clearly seen in the celebration of feast days, of which the medieval church had many. On feast days, peasants would not have been expected to work (and the medieval church had the political and social power to see that its expectations tended to be met more often than not). So the medieval church did provide a certain counterbalance which would not have been seen (at least not to the same extent) in the American example.