... is that the straps could be more comfortable. Mind you, way better than the other two primary bags I'd considered. But rather less so than other bags.
I've identified the issue: the straps are dead flat. So inevitably, given the width of a strap, one edge or the other presses into your shoulder because the bag will tug more on one edge than the other. Certain other bags (for example, The North Face) don't have this problem as much because they have rounded undersides that are somewhat softer foam, allowing for the strap to poof out on you regardless of the angle of stress. You never get an edge pressing into you. These bags also have a bit of stretchiness to them as well.
Such features also tend to wear out the strap faster, so I understand why they're not implemented on the Brain Bag. Which brings me to my second point: this is a modular bag. It has modular waist straps. Why doesn't it have modular shoulder straps? Attached with 2" gatekeepers -- or heck, oval screw-type carabiners -- up top and 1" gatekeepers below. With the right hardware design, Tom Bihn could sell all their bags and straps independently. Want a more comfortable shoulder strap? By their Ultimate Shoulder Straps. Need a more durable strap? They can provide that. Want to change the color? Etc.