From what I've seen, once you get past the department store crap into quality stuff, most of it will last for a good while as long as you keep it maintained. The higher quality stuff is usually lighter, operates a bit better, and is sometimes stronger, but depending on the part, can actually wear out faster because it is designed to be lighter weight. Get something quality and keep it maintained and you should be in business. That being said, suspension is an exception to this. Cheap suspension is just that, cheap. Get something quality that will last and is fixable when it does break. Some of the low end forks are basically use then replace when worn instead of being repairable. You will notice a difference between the cheapest bike at the LBS and a higher quality bike, but it really depends on what you are using it for. For fire roads and smooth single track occasionally, the cheap Trek will probably be fine. If you get into more serious stuff, you may find yourself wanting more.