Most of the description we get of the Miller is intensely physical and kind of, well, disgusting. He's huge, with a red beard, wide black nostrils, a gaping mouth, and (gross-out alert!) a wart on his nose with a tuft of hairs growing on it that are as red as the bristles in a sow's ears. Many of the Miller's activities are physical as well: he can break doors open with his head (side-note: why would anyone want to do this?) and always wins the ram, or top prize, at wrestling matches.
Disgusting physical appearance aside, we also get the feeling this guy's not a great miller: he regularly steals corn from his customers (probably by mixing filler into their sacks) or charges three times the proper fee for it.
The Miller's portrait draws heavily upon negative medieval stereotypes about lower-class people. The idea was that such people were "all brawn, no brains." This intense physicality was associated with extreme lustfulness, and in the Miller's portrait we get a hidden clue that this is, in fact, the case: medieval symbolism held that red hair, which the Miller's portrait mentions twice (most notably in that shudder-worthy description of the wart), was a sign of a lustful nature. The tale the Miller tells, a bawdy story about how a carpenter's wife cheats on him with a clerk, confirms the Miller's lustful proclivities. Yet the Miller's tale is also immensely clever, concluding with what literary types agree is one of the most successful and witty endings of any tale. So, like the Wife of Bath, the Miller's character questions as many stereotypes as it draws upon.