You don't have to be a professional downhill racer to know that a little riding can lead to a lot of bicycle-seat soreness. Mountain bikers who go for shorter rides may be no worse for wear, but for those who are going on long-distance rides, the discomfort caused by too much time in the saddle can be nearly unbearable. Also, the inevitable presence of sweat and friction is going to lead to some major chafing. The answer: cycling shorts (or, for winter riding, padded tights). Cycling shorts have pads in the crotch area that greatly alleviate the discomfort of riding and help prevent chafing. Men's and women's shorts have different padding designs, so be sure to choose gender-appropriate apparel.
Instead of stopping to access a backpack each time a rider wants to grab a single bottle of water (or taking hands off handlebars to access water from the frame-mounted water cage holds one bottle at a time), a mountain biker's water needs can be better met on the fly by using a hydration backpack that allows the rider to wear a specially designed pack with a large water-bag insert. This can be worn over or under a shirt. Hydration packs generally carry 3 liters (about 3 quarts) of water at a time. A tube with a drinking nozzle that winds its way from the back of the pack over the shoulder and down the front of the shoulder strap makes it possible to re-hydrate without needing to divert your hands, which have important matters of their own to tend to on the handlebars.
Finally, comfort comes in the form of peace-of-mind when you utilize a GPS satellite navigational system in your mountain-biking gear. GPS systems come in a variety of forms (wristwatches, smart phones and bicycle-mounted systems) and are more reliable than a long trail of bread crumbs when your extreme ride has left you extremely lost.
Next: Nothing prevents tires from going flat, unless of course your tire incorporates modern materials normally used for stopping bullets.