I'm assuming you don't mean a dedicated freight or industrial bike with it's oversized cargo platforms nearly 400 lb cargo capability
So, what I would look for in a dependable cargo/commuter bike:
Either a dedicated touring frame or mountain bike frame. Something designed to take a pounding.
Gears, a wide range of gears with the lowest in the neighborhood of 20 inches
A handlebar with multiple riding positions. Either a MTB bar with multiple position bar ends, or a conventional road/touring drop bars.
Medium wide comfortable tires. I was partial to commuter or centerrridge AT tires. I used to commute on high pressure slicks and went to slightly wider tire at a lower pressure. I did loose a fair amount of speed, but I didn't slide as bad in the rain and my <ahem> no longer felt like a maraca
Panniers and a rack in back and a basket in front for stuff you want to keep and eye on. Be careful about loading the rear rack too high or you may find yourself unable to shift off the back of the seat during a hard stop.
Fenders. The worst part about riding in the rain is the water thrown up by the tires.
Chain or chain ring guard to keep your pant cuff and shoelaces safe.
Pedals that can handle street shoes as well as cleats(if you use them)