Without any reference to what others have written yet:

1) How to dress for the environment. I'd say exposure to the elements is the #1 danger in hiking. Good clothes should be the core of a person's hiking equipment.

2) How to stay found. Learn to use a map & compass, GPS optional, and pay attention to where you are. Getting lost often happens when people put themselves in situations over their heads; maybe they think the trail is easy to identify and they go hike unprepared, maybe they've been hiking for years without mishap and decide to go someplace new without a map.

3) How to stay hydrated. Not a skill per se, but a lot of people don't drink enough while hiking, or don't plan right and run out with no means of replentishing. This can fit in with mapreading, to; you should be able to plan your route with sources of water in mind.

Other skills that have merit: first aid, staying fit, fueling your body right, having good hygiene, knowing not to leave garbage in the woods. It's also good to be a leader, and by that I mean that one should be responsible, resourceful, and knowledgeable. This counts if you're in a group or by yourself.