Your BOB should address basic survival requirements - food, shelter and water.
If you're too lazy to make your own BOB, there are some commercially available, like the Safecastle Rig and the Go Bag Survival Kit.
In Sydney, Australia, Mayor Clover Moore is advocating citizens create a 'go bag' - the idea is not new, and has been mooted by US authorities for its citizens. The Go Bag recommended (see picture) is just about the poxiest piece of kit you could ever devise (including the 'disc containing back-up files of work' - your life is at stake, stuff the paperwork!), but it's a starting point - especially for women wearing high heels. Commuter 'Go Bags' will be much smaller than the list below, but stick to the basics - first aid, water and food - and build from there. Consider having multiple BOBs at work, home and on your person.
BOB elements and sizes vary, but if you want to be prepared for an emergency, think about including some of the following:-
Fully-charged Mobile (Cell) Phone - the network may go down, but if it doesn't, you can still contact family and friends.
Personal papers of importance
$100 in cash
Family photo(s) in the event of a disaster, for use in the search for missing people
Maps (topographic) of your work and home area - if public transport fails, you need to know alternate routes/ways to get home.
Compass - and make sure you learn how to use one
Basic first aid kit
Dust masks x 2 - try not to crush these as you pack them as it makes them less effective
Water bottle (full size - enough for a 72-hour period)
Water purification tablets for when the good water runs out
Torch with fresh batteries
Knife - Swiss Army at the very least, or a Leatherman tool in knife/gun-phobic countries such as Australia
Radio with fresh batteries (rotate batteries regularly - old batteries are great for remote control handsets)
Antiseptic gel handwash
Aspirin/Panadol - or headahe/painkiller equivalent
Beanie (in the winter, it's important to keep warm - you lose a considerable amount of body heat via your head)
Tissues or baby wipes for cleaning/personal hygiene
Duct tape - has 101 uses and is indispensable
Binoculars are a handy piece of kit that can also help you find out what's going on around you.
If you have room (and depending on how big you want to go) a cheap tarp and some rope is a great addition.
In an emergency, your core objective is to survive, and then to relocate to a safer area - usually your own home. Don't hang around in the city. Help those that you can, but your chief aim should be to get to a safe place and regroup with family and friends. From there, if it's necessary (ie. in a natural disaster situation), plan your next move.