Thinking about a 72-hour bag for reasonably temperate climes my thoughts start at "the rule of threes" and go like this:

• Shelter: At least a Heatsheets blanket, some cordage and a knife, along with wearing clothes appropriate for the location and season
• Water: By far the heaviest part of what I would have, in addition to bringing clean water I would also want to have chlorine dioxide tabs, a small cookpot, and firestarting materials
• Food: Not essential for 72 hours but awfully nice to have, there are a lot of options here but try to find something that isn't terribly thirst-provoking
• Communication: A charger for your cellphone, a PLB or a SEND, this is the thing that if it works keeps you from needing all the other things, or at least needing them for as long
• Knife: At the minimum a good folding knife and a good multitool, preferably add a fixed blade knife as well if you're going to do any serious shelter-building or fire-starting
• Fire: Lighter, matches, firesteel, tinder, redundancy is cheap and light while not being able to start a fire may end badly for you
• Flashlight: After communication, knife and fire this is the most important thing that I think is really hard to improvise, have a good one
• Navigation: Staying put once you're lost is usually best, but having a map and compass may keep you from getting lost in the first place

Altogether, not including appropriate clothes and footwear, but including half of the weight being water, you're looking at around twenty pounds. Keep in mind that you may need to add more water, or a sleep system, or a tent, or other things depending on where you are and what you're doing.

EDITED TO ADD:

My Internet was flakey when I posted this. That's my excuse and I'm sticking with it.

Add first aid, sunscreen, bug spray, and duct tape.