Bivvy bag, tarp, and changes of clothes, including a 100 percent wool outfit and polypro underwear for those cold nights. I'm still tossing around whether or not to take my woobie. They make awesome pillows, but degrade pretty quick.
Tents and sleeping bags are too heavy! I got a lot of other crap to carry in that thing! Maybe if I had a bugout mule come with me. :
I use a Eureka Chrysalis 3 season Hammock and a wool blanket... I dont like keeping my sleeping bag rolled up for any length of time other than getting to and back from my camp site - its a nice bag and I dont want to screw it up.
Your clothes can be most of the heat. A very efficient, but water resistant sleeping bag can be used.
In the old days, the guys used to light fires and bury the coals. Then they sleep on the coals overnight. That plan would work great, but I am weary about a large enough fire to make those coals. I want to keep where I am as private as possible.
With as small as the 1-man tents can collapse these days, it's probably a feasible thing to have a small tent in your BOB. I have a large 8-person tent that will go with me if I have time, but I've definitely been thinking about getting a 1-man to just keep in my BOB or in my truck 24/7.
If you have room in the bag carry both.
I have 5500 cubic inch pack with everything for at least 4 days, including food and water.
I do not have to remember to add anything if bugging out for less than 4 days.
Kev's idea about a bug out box is a good one, I have a couple of large plasctic bins with food and gear for medium length bug outs, up to 4 weeks, I even keep extra tent and sleeping bag in them too.
I keep as many of my supplies as possible in bins and large duffel bags so it is ready to go.
Both. I have to deal with temperatures from about 110°F down to about -20°F. Some times even wider extremes with winds from dead still to 80mph. A tarp just doesn't cut it for in 2 feet of snow, 0°F, and 40mph winds. At least not for me. I like the idea of a hammock, especially for summer, but I'd have to carry one with a frame with me to set it up, or a couple miles of suspension cord. Trees can be very few and far between out here.
I think for most people it depends on weather conditions. No way you would live here in sub zero conditions with no bag, tent, shelter and so on. In summer the bugs are pretty bad, although they don't bother some folks.
I like a small, tough tent, a light bag and a sleeping pad.