I forgot that i have a pair of gloves, and i didn't mention the auto specific stuff like the bottles of fluids, jumper cables and road flares etc.
Well, the extinguisher issue is a bit of a problem, it's difficult to keep anywhere else than the trunk since it is about 2 feet long. I agree that having it in the passenger compartment is better, but there's no good place to bolt it, it's not a race car with roll cage.
Good point with the nails. I should toss in some 10d and a selection of drywall screws. Also ty-wraps. This would live in the kit bag with the rest of the small stuff though the big stuff is just pinned behind a milk crate. Portability was not really considered since, in southern california, you are probably near your car (both at work and at home) and is more likely to be accessible than a box buried in your house.

Also, i suppose that depending on your area, other tools would be useful, say snowshoes in the north.