Motocross gear, unlike motorcycle gear for street riding, has a shorter life due to the use and abuse it gets put through. When you go out to hit the dirt, not everyone likes to admit it, but sometimes you literally "hit it," and that fall can tear holes in your jersey or scuff up your riding pants. Whereas motocross jerseys can afford to be thinner in material (for ventilation purposes), the motocross pant is what is being worn down from constant contact with the seat and sustained abuse from the dirt and rocks as well. When you shop for motocross pants you should keep these three qualities in mind: lightweight design, flexible material, and durable fabric. A splash of color doesn't hurt either.
When light weight is one of your priorities for the race, any additional material to the pant is the obvious key factor that could add ounces. Take the O'Neal Mayhem Crank pant, this design favors light weight, so the company nixed rubber thigh patches to reduce the heft. Further, it reduces weight with the lighter, more breathable panels that are positioned away from the bike's contact points.
Another feature to be mindful of is the flexibility of the pant. You will be bending and extending your knees frequently and moving at the waist when you shift your weight on the dirt bike. Stretch and flexibility in the material in those key points will lend to overall comfort both on and off the bike. Stretching cuffs and elastic bands help, as do some multi-directional stretch-rib panels as seen on the Fly Racing Kinetic Mesh Era pant.
Durable material is key because, as stated before, the pant is continuously sliding along the motorcycle seat during aggressive riding. Those active motions tend to rub the material frequently and thus cowhide leather panels and 900-denier polyester, as seen on the Joker SE Pant from Troy Lee Designs, can help the product last longer and keep you comfortable. The higher the denier measurement, the thicker and more durable the material will be, and cowhide leather in key placements such as the inner knees can be important.