The mountain hubs I have seen tend to have longer axles, to reduce wheel dish. For example, a standard 8- or 9-speed road axle has a 130mm overlock dimension, whereas all of the 8- or 9-speed mountain axles I have seen are at least 135mm overlock.

Mountain bike hubs may also have heavier-duty bearings and thicker spoke flanges, but any high-quality hubset with the correct dimensions should be suitable for the typical non-racing cyclist.