Where Rome had a heavy reliance on slavery, medieval Europe had a heavy reliance on serfdom (in the case of eastern Europe, lasting up to last century).
True, although the lot of a serf, at least in the West, was better than that of a Roman agrarian slave, IMHO. Plus, serfdom was introduced fairly late in the "Dark Ages" and in the West began to fall apart after the Black Death. Agreed serfdom lasted into the 19th Century in much of Eastern Europe, but that's one reason they were so far behind economically.