A BOB's contents are dependent on the situations you are likely to encounter in an emergency. Climate, location and type of emergency are all relevant. I see a lot of folks say Bug-Out Bag and then go on to describe their 72 hour kit. If you really think it through the two kits aren't the same. My 72 hour kit is really more like a 96 hour kit for two people and it stays in the back of my truck, no way could I carry it. As part of the larger kit, I have a bag set up for survival camping and another smaller bag for walking. The walking bag is empty until I decide what I need from the 96-hour kit to get to where I need to go on foot -- this is plan "last". I would prefer to stay with the truck, but if need be I can park it and go for a walk.

In the plans I've seen, one thing that a lot of folks forget is that when a massive power failure (hurricane, earthquake) happens a lot of vendors can't take credit cards, so have some cash in your BOB -- how much is up to you, but have enough in small bills to pay for stuff like food & water while you're walking to wherever you need to be. BTW, where are you going?

Essential to any bug-out plan is a bug-out location/goal. How much you need to carry is dependent on how far you need to walk. The farther you need to walk, the more you need to carry and the more you carry, the shorter distance you can walk. You really need to think this through.

My plan is to Bug-In if at home. I have all the supplies of my home (food, water, shelter) and the 96-hour kit in my truck. It would take a wildfire jumping the fence to force me to bug-out from home and in that scenario, I'll have a few days to decide what will go with me and to prepare the truck for a long drive. I'm evacuating in my truck with lots of stuff, not bugging out on foot.

BTW, Wikipedia's Bug-out_bag page has some info and there are some useful links at the bottom.