Simplistic answer. Road cleats extend below the sole of the shoe. Mountain cleats are recessed up into a pocket in the sole..
Beyond that there are many brands and styles that become matters of personal like and dislike.
I have a handle on the differences, I am curious what makes the road shoes superior to the MTB? I toured for years wearing just running shoes. This year I went to a clipless pedal, and bought some Nike MTB shoes. I have ridden with them a couple of times so far and really like them. I see where some feel any distance over 50 miles and the road shoes are far superior, and am just wanting to know what those differences are. Are the shoes lighter, cooler on the feet, or what? Just curious. I might want to switch at some point since I do quite a bit more than 50 miles a day when I actually tour.