The features that I really keyed in on were the head shape and the bill. Consider the crown of the head. On a Ross's Goose, the crown is rounded while on a Snow Goose or hybrid, the crown is more elongated. I would say that this bird has an elongated head shape.
Now look at the bill. The bill appears to be intermediate between a Snow Goose and a Ross's Goose. It lacks the "grin patch" typical of most Snow Geese. It has blue coloration near the base of the bill like a Ross's Goose. And the transition between the back of the bill and the facial feathers is vertical like a Ross's Goose. In addition, the bill proportions seem slightly longer and shorter than what is typically seen on a Ross's Goose. These intermediate bill features once again suggest a hybrid.
But just because this bird is a hybrid doesn't mean that it's parents were a pure Snow Goose and a pure Ross's Goose. This bird could very well be a backcross, which means that one of its parents was itself a hybrid. Whether this is the case, I don't know. But what I do know is that studying this bird and reading about hybridization between Snow Geese and Ross's Geese has been a real learning experience.