Originally Posted by multisport

I know I will miss some things, so others please feel free to jump in.

One difference is the gearing. Mountain bikes generally have lower gearing than 'cross bikes. The gearing on a 'cross bike is inbetween a mountain bike and a road bike. Most of the time, one won't need the granny gears of a mountain bike.

Other differences: less rolling resistance of the tires since they are narrower, lower bike weight, and the riding postion help provide "an advantage."