For us, they needed to be small enough to carry-on (this meant about 46 liters or smaller), they needed to be durable, comfortable, and we wanted straps that stowed away in case we needed to check them because of weight on smaller flights in Asia.
Extra features that we wanted were padded laptop compartments, water bottle holders, detachable daypacks, nice suspension, and usable organization compartments.
It has a lot of padding, feels durable, feels safe to check onto the plane, and can attach an optional Osprey Daylite pack to the outside (although attaching it is pretty clumsy and callus-inducing on the fingers).
They have a lot of neat features, great organization, a water bottle holder that packs away (it will not fit a Nalgene – only a disposable water bottle), an inside shelf to keep everything from shifting to the bottom, a special pocket for quick access to liquids, and many more fun organizational features.