MTB upgrades and components such as pedals, shoes, handlebars, tires, stem and wheel set can have a huge positive or negative effect on your riding! Choose the components and equipment for you, the conditions and your purpose that day.
On a technically easy course like the 24 hours of old Pueblo a racer will be concerned more about rolling resistance than control so she runs a semi-slick tire front and rear. The same racer on a more technical course would likely run a knobbier tire that rolled a little slower but gave her more control.
Some things to look out for:
1. Light weight components, light bars, light wheel sets, light tires, light cranks etc.. Our obsession with shaving weight off our bikes needs to end. Yes, given the exact same performance I would rather have a 22 pound bike than a 32 pound mountain bike but right now that doesn't exist.