It must be a backpack - shoulder/messenger-style bags give me a bad back.
It must have a compartment that provides a decent amount of protection - I'm spending a lot on that laptop, after all.
The protective compartment for the laptop must be designed for a 15.4" laptop. I don't want it shifting around in the bag.
Those are the non-negotiable points really. The amount of extra space, beyond holding the laptop, is something I'm not sure about. Because of the amount going on the laptop, I am also willing to spend a decent amount on the right bag.
I have done some looking around, but I've not found a lot of decent reviews; maybe I'm looking in the wrong place. I am at the moment leaning in a particular direction, but I'll keep that under my hat, at least to begin with, to see what recommendations come up.