I retired out of the Army about 2 and half years ago. Spent a lot of time in Iraq since 2001, also spent 9 months there back in Desert Storm.
We'd spend a lot of time moving around and living out of what we'd call a "3 day bag" since we were mainly pilot's we would only spend up to say a week out from the FSB (Forward Support Base).
It was always a hoot to see what people would think that is absolutely necessary to have in their bags, and what you'd see them pull out.
We didn't carry rucks because we didn't have to stay out of the wire that long. That said;
If you were to leave the house today with a "3 day bag" for the purposes of this discussion, it would be a bag that you could easily put on your back or carry by it's handle. It would be a bag that if you had to abandon your bike or car, you would grab the PC (Precious Cargo...wife or girlfriend) and that bag, quickly, and have what you need for the afternoon or a day or two.
It has to be of a weight that can be carried for some distance.
I have lots of ideas but I will wait to throw them out as the bag builds.
It's obvious that a cold weather bag would be different than a warm weather bag, you may delineate between the two.
example; you don't need mechanics tools, you're leaving your machine, but a Gerber tool or the likes would be important.
What's in YOUR bag?