This is the best article I have seen regarding MTB setup. Some of what you see in the World Cup fits are absolutely awful.
Excerpt:
Wider bars also allow you to keep your arms bent and chest down allowing you to ride in a more athletic, neutral position. Perfect for riding smoothly and adjusting to anything and everything the trail throws at you.
Your stem is a not a bike fit device, it greatly effects the control of your bike. Motorcycles don’t have stems for a reason, a long stem puts you out of balance (too much weight forward) straightens your arms (taking you out of a neutral position) and the long lever of a stem more than 90 millimeters long makes your steering “flop” to the side instead of being precise.
So for a more controlled ride go with a 50 to 80mm stem and 27″-32″ wide bars. I know this goes against tradition so please try this set up for a week before commenting. If you understand correct body position, how bikes turn and how to manual or wheelie correctly (using no upper body strength) you will love the control this gives you.