(Note: "mountain bike" (MTB) is not equal to downhill. The bike mentioned here is a XC mountain bike, intended for speed, not trial-like cycling.)
As with any bike, or "mechanical thing" for that matter, things tend to urge for maintenance after some time. To me, the overall best way to get things done, and hopefully also save some money, was to do the work myself. And now, 7 years later, this argument seems to hold.
Well, when it comes to fixing and repairing yourself, you must have quite long-term visions either 1) for your bike, as tools required may fit this bike only, or 2) make sure your bikes doesn't differ too much, so tools may be used over again. If you need (new) special tools for each bike, than you would have to quit your job and put up a bike repair shop instead, as this becomes awfully expensive.
One of the first things you would notice about disk brakes, which is an advantage itself, is that you won't have to replace brake pads as often as with regular rim brakes. I have changed my ceramic pads at most 2-3 times in all these years.
(The Giant MPH brake system has this interesting oil house at the bar, as seen above, where the hydraulic pressure may be adjusted)
After brakes, chain and chain rings would be the next thing to fix. I learned this the hard way, as I wasn't aware that these things actually wear out! But you readily know this when the chain keeps "jumping" when pedalling - then the chain is worn out. And chain rings should be changed at the same time. A chain cutter will be handy for this work.
During biking (or bike handling) you might experience that spokes snap off, and needs to be replaced. I you're a dedicated biker, you should probably seek some professional help here, to have the spoke replaced and wheel tuned properly. But I have had good experience in using a spoke wrench myself.
After a few years, your gear and wiring probably needs to be replaced. At least the rear shifter tends to cause the outer cable to fail, and must be replaced. This is a simple operation, but beware to start in the right end, and mount ferrules and outer cables in the right time
The crank (bearing) is perhaps the one I have changed most frequently. This started off when the bike suspended while one of the pedals were at the lowest - and the crank actually broke.
This is actually the part for which I have the most special tools, which includes:
- Shimano/ISIS standard crank tool (link)
- Pedal remover tool (link)
- Second standard crank removal tool (link)
The latest repair I did was to the unique NRS suspension itself: as I disassembled the bike for cleaning and inspection lately, I discovered two bearings in a very bad state. Fortunately I could read the bearing seal, saying "688 RS". And I found this bearing at jensonusa.com, and got 4 of them (though only 2 are actually needed for a complete replacement).
This bike will be agile for many years to come