72hrs is a pretty good standard and every load out obviously like some said is mission and environment dependent.
My 72hr blowout bag would be for rural blowouts IE: things go bad and I have to survive or live remote for a minimum of 72hrs.
Fundamentals.
Food and Water.
I use REI dehydrated chow broken down, MRE's are good options but from my military experience are to damn heavy, accumulate to much trash, and are grossly high in sodium and less in real nutrition. my military brethren would agree... if you start eating MRE's you will shit bricks, be constipated, and it will suck the water out of you.
I would stick with nuts/berrys, real good hippie granola, high in fiber, protein, and dry fruit high in calories, and carbs.... sustaining nutrition.
Next, water-which is the most important. I would bring at least a 200oz bladder filled with water, a good nalgene bottle (for decontaminating and portability),
and either iodine tablets, or bleach to decontaminate water. Part of my food and water kit will include a small pot to boil water, and I prefer fuel tablets, and lighters...
this setup is light, avoids fuel signatures like smell, and allows you to boil water with a reduced signature... also good for morale to boil coffee, or heat up chow.
Next I would obviously bring things for survival... alot of people will tell you pack out wire for snares, fishing line for catching fish, and some pretty crazy survival tools that really don't work... I have been to SERE C (War time), SERE 215/245 (Peace time detention survival schools in Special Forces, and will tell you that this shit doesn't work... When I was in SERE school evading, me and a buddy rapped an old ladies garden for food, and that works... to try and bring these micro kits for survivability is a waste of space.
I would bring a leatherman that has a tool for everything, if you're going to carry the weight of a knife you mine as well bring a leatherman with multi tools.
Some guys will say, carry a wire hand saw, I would actually bring a hatchet... worth the suck up in carrying weight if you ask me. Winkler Axes and Hatchets are the very best. I have used his tool to rip open cars for rapid entry and they hold an edge like no other hatchet on the market.
The next issue which is overlooked is hygiene and medical. I would pack out baby wipes, motrin, caffeine pills, 4 X tourniquets, hand sanitizer, balm for hands and lips, foot powder, sun tan lotion, surgical need and thread for sewing stitches, alcohol wipes, soap, floss, and most importantly antibiotics for both bacterial infections, and viral infections... human beings are walking shit pools, guys who have been in the field for extended periods of time know this... your hands fall apart, your ass gets chaffed, and your feet start rotting.
Socks, socks, socks... if youre making significant movements across land you will need to take care of your feet, this includes keeping feet dry, and changing socks frequently. I would also pack an extra set of underwear and shirt.
Sleep/Shelter-obviously you need to rest, and you need to seek shelter out of the elements. I would use a compressible stuff sack and use a sleeping bag shell that is used to keep in heat, and join that up with a poncho liner. this will keep heat in and protect from the elements. I would also buy a small tent, some guys go old school and use ponchos and 550 cord, but now a days, you can get a nice compact tent thats lighter than 550 cord, and ponchos... also, problem Ive seen with ponchos is that they actually don't wick moisture... again if you grew up in the military and ever went to sleep with a poncho, even on a dry night you will wake up wet, because it sucks up humidity...
Day/Night signal
I always pack out VS-17-pretty universal, signal mirror, some chem lights with 550 cord to make signals you can swing around to friendly Identify.
Navigation
Depending where you are GPS like a garmin 401 is a must, I prefer a GPS that I can load map data, and has the longest life. Again some guys would say maps and compasses, which aren't bad ideas, but if I'm carrying the weight I would pack out the batteries, and maybe a solar panel pack that trickle charges your electronics.
Hope this helps.