The ability to observe and understand what you are observing, then take appropriate action.

The ability to navigate.

The ability to actually use what you have brought.

edited to add: I have been taking small groups to national parks which are rife with people who go out maybe a few times in their life, carry too much, don't enjoy the actual act of backpacking so much as they want to go look at scenery. I see people get sick, sore and hurt because they are not observant of their bodies or their environment. They don't realize they are walking three inches from a rattlesnake, or that the half liter bottle of water will be inadequate for the six mile round trip they are on. Backpackers will bring gear in its original store packaging and fight with a stove that came with a piezo for an hour until I loan them a match. I have walked up to someone standing on a rock ten feet off the trail and given her directions back to the trailhead, half a mile away. She didn't see the trail ten feet away. 90% of the backpackers I see out there don't know, don't care to know, and don't ask - backpacking is intended to be misery, isn't it? That's part of the deal.

I'm no great compass guru, but I can navigate by landmarks and shoot a bearing. I can see on a topo map that a route will be pain and suffering for my handful of day hikers. I am already at a better advantage than the dudes with 100 lb poorly fitted packs who go out without a map.

Observe, plan, experience, reassess, tweak the gear list, observe, plan.... I just did a trip I had gone out on a year ago, and back then it was stress and weariness and not very relaxing. I head out on the same trip this past weekend and come back feeling refreshed and ready for more, even though this time around I went out bushwhacking and climbing steeper trails. My gear list has been evolving and it all works better now, and my body is better conditioned than before; I also know and respect my limits now. The gear is only part of the whole. Skills and adaptability are the more critical pieces.