Originally Posted by
never
Geometry and the amount of travel are the biggest differences. Geometry maybe even more than travel.
The bikes are heavier because they take a beating and lightweight XC parts wouldn't hold up as well (though some DH parts are getting pretty light).
This is the key... a lot of XC Trail bikes have like 5-6" now... the suspension systems have gotten advanced enough to make that palatable. The problem remains with geometry. Head tube angle, seat tube angle, handlebar height(stack) and the top-tube/stem length(reach) are what make it hard to slowly work a DH bike uphill but they're also the things which make it better at downhill. If you sit on a DH bike it becomes obvious it's designed to only go downhill...