I would say that the major difference is style though there are a couple of differences. For example, road helmets tend to pay specific attention to their profile to limit vision obtrusion's when in the drops. They tend to have more vents to keep you cooler with the higher speeds of a road bike, and in many cases, spending more can get you a little bit of a more comfortable helmet.
The practical truth of the matter is that most people get "road" helmets because of style (one that I admittedly prefer and use for both road and mountain riding). As long as you are comfortable with your helmet, you have the right one.