Skin color and (facial) bone structure for example. Like it or not, but humans have adapted to living in different climates and have different ancestral lines. Take a picture of an average black person and edit to white skin, it will still not look like a white person.

As the commenter above you already said, this is not a bad thing. People on _hacker_news should be able to understand that being able to define the ==/!= operators on something does not automatically mean that you can define < and >.