1) Map/Compass The AT maps are awesome, don't show up with a 1/2 inch compass - too many get turned around in the morning.
2) Hydration (water) nope - Sawyer Squeeze. & a empty soda bottle with the matching thread.
3) Food - HB eggs, Home made Jerky, or Cliff bars or your choice.
4) Rain gear No see number 10 /extra insulation No buy better Fleece please. The best is a down Jacket with hood to supplement your bag.
5) Fire starter Micro Bic (White) & if you are really lazy or have a cheap knife. Add a cube of Esbit.
6) First aid (Imodium+pain relief) perhaps a blood clotting patch. Toss th bandaids, that should be your sham wow or neckerchief.
7) A locking blade knife - leave the small knock off Swiss at home.
8) A really good headlamp I prefer the Microstream
9) DWO insect wipes, Sunscreen - debatable. - its a long green tube.
10) You may already have a tent. so no a Poncho with pre-made reflectix guys and Ti stakes, when the tent gets smashed, or falls apart or you need a vestibule.
Missed - Storm Whistle (awesome)
Missed - Short hikes in groups I carry a $1 disposable poncho for those that forget. This doubles as a vapor barrier.
Missed - a Micro star flash... just me... I like it.
Missed - really good wax earplugs, (Don't ask)
Missed-one really large garbage bag wrapped tight. Possibly a 20 or 15 gallon for inside your bag.
Oh leave at home - any tube tents, wire saws, cheap whistles, goofy Mylar blanket anything, most electronics, close to nature bug spray, large survival knives, anything that is orange and has Bear Grylls labeled on it, & useless Bear Bells. Hard Candy - Hershey Kisses instead.
Avoid duplication, Avoid single use items - embrace light or dual use.
In the past many here got messed up about the Bic. I have heard from a leader that the BS avoid the Bic as the boys tend to flick it too much hence the matches. I have heard too many stories of Bic failure in wet conditions - you can soak it in water pull it out blow for 1/2 a minute and it will still light. Otherwise go with John Wayne's Wind Matches. REI's are wonderful.
Hope that helps - if not quote me and I will be happy to share answers to my thought's.
That's enough for now.
Enjoy.