I would love to be able to filter by number of people in an image; one, two, or group. This would help significantly when looking for a portrait or a more active group image.
I would also like to be able to exclude things with Boolean terms. For example, I rarely use beach images but they come up a lot in my searches. I would love to be able to add "NO beach" or "NOT beach" and have that filter out those images in my search.