as far as I can tell the difference is having a really really heavy bike and not being able to climb and thus removing one of the benefits of bicycling which is aerobic exercise...
I am not a dh'er either, but a admire their skill. I've ridden with champion downhillers and they
rock uphill ... even on really heavy bikes. Take a gander at John Tomac or Nicolas Vouilloz. I grant you this, there are some real turkeys out on the trails riding $4,000 downhill bikes.
