Other than the roles in life (dharma), what makes the castes different from one another?

Inequalities amongst castes are a part of the ordained natural order and it is explained or justified in terms of purity and pollution. High-caste status is associated with purity and low-castes status, with pollution. Those born into high-ranking class would have inborn purity, like the Brahmans and similarly, the low caste Dalits are born into pollution. The Dalits can easily pollute a Brahman just by physical contact and the latter will have to perform ritual cleanliness like bathing in flowing water and changing clothes. This explains for the distance the upper and lower castes maintain in terms of intercaste relationships.

One of the theories of purity is that when a person is alive, both his body and soul are connected spiritually as one. When any part of the body leaves the whole entity, its connection to spirituality is broken and it is considered impure. Polluting. So anything emitted from the body is considered polluting. For example, corpses, manure, dirt, bodily fluids, etc. Similarly, anyone who deals or is in contact with such impure matters is considered tainted and polluting too.

Low-castes that hold perform menial jobs that deal with burying corpses, killing or skinning animals, cleaning toilets and clearing rubbish and body excretion, are considered tainted and polluting. They are kept away from any sacred learning and ritual and away from the mainstream society. The Untouchables have to live outside the villages and have their own wells so as not to pollute the rest of the people.

Since occupational specialization is believed to be in accord with the ordained order of the universe, and since purity or pollution is traditionally hereditary, low castes who hold demeaning jobs have their lives remain polluted forever. For example, a Sweeper will be a Sweeper for his entire life and his children will have to inherit the same job and be polluted as well. Similarly, the upper-class groups like the Brahmins will always retain their inherent purity from their parents, as long as they carry out the daily cleanliness rituals and do not violate any purity codes.