Mostly, It's the venting that makes it an off road carb. There is usually plumbing that extends the bowl vent up, so it doesn't slosh fuel from the bowl into the venturi durring bumps and unlevel driving.
Also, on an Edelbrock, carb, there are "off road" needle valves available. They have springs in them that keep the bouncing floats from dumping in too much fuel on washboard roads. But they really don't help at all on unlevel driving.