Road pedals accept a large cleat that stands proud of the sole of the shoe, MTB cleats are designed to be inset into the shoe.
The road shoes give a big contact point between shoe and pedal, this may help to increase power (althoughI tend to think the stiffness of the shoe has more bearing here), and also to reduce the chances of "hot foot", a burning sensation on the base of the foor (occasionally) caused by the small contact point of mtb shoes.
MTB shoes generally have a recessed cleat and a sole with a tread so you can get off and push when the mud gets too deep.
Either system works well - although I'm not sure i'd want to go offroading in a pair of SPD-SL road shoes