Howzit

In part yes. Phill and Paul have a habit of saying things like "he is the best descender we have ever seen" and they say that every year about anybody who can go downhill fast.
As for the mountain bike thing, no not really. The technical qualifications vary vastly. Road bikes are more like 500cc road bike racing; the brake thresholding and apexing. Mountain biking is more like moto-x/BMX, where it is not so much the way you lean/sweep around the bend, but more of how you are able to throw the bike around independent of your body.
The part from mountain biking that does however help with road racing is the agility to correct the bike should something go a little off. Thats the part where you can control the bike independent of your body, like snapping your foot out to avoid a crash, or like how the yellow jersey today hopped into the parking lot. Mountain biking specifically requires you to be able to control the bike and your body, road racing requires your body and bike to become "one," so as to swoop through bends and curves like a car.

As someone said, the best thing that makes a good descender is familiarity with the terrain, or experience with similar terrain at least. Oh and ballz, nice brass ones.