I would say that unless I am doing an urban hike (ie…Cougar/Tiger Mt's in the Issaquah Alps) my day pack is pretty much identical to my backpacking kit.
The items not in it?
Sleeping bag
Ursack bag
tent
sleeping pad
I carry EVERYTHING else. It isn't like it weighs that much, since most of my gear is lightweight to start with.
My day kit:
Daypack
pack cover
Prolite sit pad
Headlamp
2L of water
Micro Pur tablets
TP bag (TP, ziploc bags, hand cleaning packs)
Clothes bag (down vest, gloves, hat, socks, jacket/pants for rain unless wearing)
Kitchen set (lighter & matches, pot, cup, spork, cozy & paper towels)
Food (bag of snacks, food bag with an extra days worth of food)
first aid kit
GPS or compass
Maps
Knife
and whatever else is in there.
For me, I don't wear my insulating layer on my body while hiking (I get overheated) so I carry it with me, for stops and when we go downhill on the way home. I also rarely wear my rain jacket unless it is dumping or going downhill (overheating thing). Could I survive being lost with my kit? Maybe. Truly it relies on the brain for that to happen. I choose to carry a bit more than some, over the years I have used pretty much everything I carry with me. I have had sunny days turn nasty, I have had simple dayhikes become daylong trips from hell. My food bag easily weighs in at what some peoples butt packs weigh in at.
Still, my pack isn't heavy by any means…and I can say it is very nice to have a hot lunch in the alpine meadows…
Oh yes, and in winter it changes:
down jacket, outer gloves, face cover, snow shovel, etc.

