I've always been a firm believer that, if it isn't on your person, there's a good chance it won't be there when you need it.

I've heard too many stories of people who put their pack down at camp, walked away from it to go take a wiz or something, then wound up lost, injured, attacked by an animal, ect.

With that in mind, there's a few items that, if carried, I'd much rather have them on my person than in my pack. These items are (in no particular order):

PLB/Cell Phone/Form of communication
Flashlight
Handgun (Pistol)
Knife (pocketknife or otherwise)
Water
Fire starting method

I believe that all these items are too important to not have readily available. Especially since items like flashlights and PLBs/Cell Phones are getting smaller and smaller, making them easier to carry all the time. A few items have even become multi-purpose. For example, my cell phone has a built in flashlight that's adequate for finding your way around. I've also seen some knife/lighter or knife/ferro rod combos (though I have no idea how good they are, I'm sure they are still better than nothing).