I live and hike in AZ so first and foremost is WATER.
A camelback and a another water bottle too. If I'm going for more than a day I bring either my water treatment (tiny little bottle of iodine tablets) or my water filter.
Other than that...
Tent
Sleeping bag
First aid kit and stuff to repair gear (small duct tape, zip ties, etc)
Extra socks, usually no other clothes unless I toss in a set of shorts to wear at a swimming hole
Tiny shammy to use as a camp towel
One pot to boil water in for freeze dried food
Aforementioned freeze dried food.
Small camp stove
small headlamp
matches
bug spray and sunscreen
a few ibuprofen, pepto bismol, and other pills