Expansion and contraction to fit your load. Expandable gusset is very thoughtfully designed; it works without you having to think about it.
Surprising amount of space for how small this thing can look on the body. Mirrorless cameras with small/pancake lenses will likely fit, for example.
Totally customizable and removable inner organization. Can all be removed and makes a great daily carry bag (or leave some in and customize for how you carry).