This subject struck me, first and foremost, as a question of philosophy rather than an expose on UL methods and gear. After reading the title of this essay several times to get my head around it, I decided to choose two different perspectives on the subject. The first perspective is taken from a very literal point of view: with an OVERNIGHT pack, what would I consider ESSENTIAL? With this perspective, I am going to equate "essential" with "survival." I will also assume "overnight" means two days walking, plus or minus sleeping through the night (as an ex-orienteer I am fully aware of the option to not sleep if it's only one night). I am going to assume that sleep is NOT essential in this case, so will leave bag, mat, and shelter at home. Likewise, it won't be necessary to cook, so I'll leave stove and fuel at home.
I may want a very small and light day pack to carry some food and water, though given the assumed scenario, even these are probably not "essential" unless I'm trekking through 120 °F desert conditions, where water might become essential over two days. Clothing would depend on the expected weather and would be mostly worn rather than carried, with the option of keeping moving if things got too cold to stop and rest. I would probably carry some firestarting supplies and a little food, just in case I got lost or injured and couldn't make it out within two days. Hopefully these last items are not essential, but I'm allowing myself a small safety margin.
The second perspective is where my alter ego steps in and decides it is "essential" to be comfortable, warm, dry, and well-fed. Hmmmm, that's a whole different game. This time of year (winter in New Zealand), I would probably head for a hut that I know has a wood burner and lots of dry firewood or coal, plus a comfy mattress. I would bring a nice down sleeping bag, a stove, pot, pot lifter, fuel, change of dry clothes, hut shoes, and some whiskey. Firestarting material would also be "essential" in this case, as would a comfy pack to carry it all. Winter nights can be long, so a deck of cards or good book might also make my essential list. In summer, insect control would also become important, as would sun and heat protection (hat, sunscreen, sunglasses, tarptent). Here, I'm obviously not using the word "essential" to equate with survival, rather it's merely a relative degree of comfort.
The bottom line is that I have a lot of survival and problem solving skills. If I were dumped in the middle of nowhere (assuming I was fit and healthy), I am confident I could survive at least a couple of days with nothing more than my brain. I hope I never have to test this theory, so I always carry a margin of safety in my gear based on anticipated needs AND desires. I am not out to push the limits of survival, nor am I out to break the two-pound base weight barrier. I am out to enjoy myself. Heck, I might even bring a toothbrush, some soap, and a hairbrush to maintain a modicum of presentability. To me, an essential overnight UL pack could contain anything, from absolutely nothing to fifteen carefully chosen pounds of comfort and security.