Unlike stone mills, steel roller mills eliminate much of the corn kernel, including the germ; doing so makes the corn shelf stable but also robs it of much flavor and nutrition. The friction of steel rolling generates a lot of heat, too, which further erodes corn's natural flavor. Perhaps the most significant difference, though, is the size of the resulting meal.
"If you're toll milling," Roberts says, "you're using one screen. It's just like a backdoor screen. If you put the grits onto that screen and shake it, coarse cornmeal is going to fall through. The diverse particle size in that cornmeal is stunning when compared to a [steel] roller mill.