Hmmm... I believe in the Persian proverb Colin Fletcher likes to quote :"Fortune is infatuated with the prepared."
As a backpacker and an ex-Girl Scout (as well as a worrier) with thousands of miles hiked, we carry:
GPS with coords downloaded
daypack with rain cover
T.P. in a plastic bag
first aid kit - mine own collection
extra pens
plastic garbage bags
heater-seats (esp. when it's wet/cold out)
bag of gorp
bottle or two of water
some light weight rope
flashlight
extra batteries for GPS and flashlight
maps, if I have them
printed out cache sheets (no decryptions, please)
space blanket
gloves, hats, scarves (if weather decides)
bug repelent, but haven't used it in a long time
bandana/kleenex
swag, extra zip-loc bags, extra log book
PLUS:
a big dose of common sense and respect for the topography we will encounter
If it is "civilized" and not backcountry:
GPS, swag to stuff into pockets, printed out sheets