It's an exciting time in mountain biking technologies. We have so many tire/wheel options, gearing options, frame designs and frame materials. It's a lot to digest and for the technology obsessed it must be quite a wallet-draining experience trying to stay on top of these new products. However for the more frugal mountain biker there are some worthwhile purchases to make sure that you're getting the most out of your existing rig.
The most important bike parts to be picky about are grips/handlebars, seats and tires. These parts govern how the bike interacts with the outside world.
Always test out the tires that come stock on any bike. Give them a fair chance, unless they are dramatically unsuitable for regional conditions, play around with tire pressure and give them a couple rides. No two riders are completely identical, so there is no ideal set up for all riders. I spend a lot of time trying to get as much information from my customers as they get from me because I'm short and a welterweight (with gear, always consider the weight of that fully loaded hydration pack when setting up a bike) so I don't have any experience taking 200 lbs of bike and rider down the trail. Whenever shopping for tires, consider the reviewer's body type, I wish I had always been aware of this and regret the first few years of my own tire recommendations to customers because it took a long conversation with an upset customer to point out my own failed guidance. About a decade later, I've tried to develop my own data bank of riders and their tire choices, every time a new product comes out the process starts over. Ask similar sized friends with similar riding styles about their preferences in tires, ask a random rider on the trail about their tires. Ask about specifics, how is it in mud? Have you ever ridden it in snow? Does it hook up in rock gardens? What air pressure do you run? Namely picking out tires takes some patience and communication skills if you don't want to spend a lot of money on trial and error. The basic science behind tires is pretty simple. Taller treads flex and waste more energy than shorter treads, however will provide more traction for a longer period of time. Softer rubber compounds grip better than harder compounds, however wear out faster. The surface size of the knobs should be chosen relative to rider's weight, if you're too light of a rider you won't get big knobs to actually dig in so you'll just be riding on the surface. If that surface is loose, big knobs will actually provide less traction for smaller riders than small knobs. Hard packed it's important for all riders to minimize tread height and gaps between knobs for maximum control. A great source of free tires to check out, are any experienced friends that might have tires they didn't like lying around, just because they didn't like them doesn't mean you won't. The right tire and pressure will allow you to get the most out of your ride.
Grips and handlebars should also be discussed with brake lever positioning. If you find yourself with upper body soreness limiting the duration of your rides, it's time to start adjusting your cockpit. The cheapest most important thing, make sure your brake levers are in aligned so your wrist is straight when seated. Secondly, bring the brake lever reach adjust (if possible) closer to the bar leaving a little more than a finger thick gap between where it stops and the bar. Your grip is strength increases as you close in on making a fist, using just your finger tips your forearms will tire out faster than if you have it so you get your middle phalanges(bone in the middle of each finger) on the lever. Handlebars are available with different rise and sweep, rise is as simple as you can imagine and is labeled in millimeters or inches. Sweep is labeled in degrees and describes the angle at which the bars curve back towards you. More sweep will relax your shoulders and rotate your elbows inward making it easier to get back over the rear wheel in rough descending. Less sweep will make a bike more nimble and help you get your weight forward for challenging or aggressive climbing. More rise will relax your lower back and put more weight on your seat and consequently your rear wheel. Grips are simple, make sure it's got enough substance to absorb the vibrations and it's big enough to fill your hand but not so big that it challenges your ability to hold onto the bar.
Seat choice is equally as affected by rider size/body-type as it is by rider position. You'll see every company has different ways to measure or label riders. Most manufacturers fit systems will function as decent guidelines but not a complete replacement for real life experience or personal consultation. Sometimes I wish I was getting paid as well as a nurse or doctor while discussing hemorrhoids and other medical issues during seat choices but I've helped a lot of people through a lot of very specific issues relative to riding comfort and saddle choices, I'm not a medical professional so I can't offer any guarantees with my advice but I'll help as best I can. The only good news in choosing a new seat is that the moderate options work for most people in moderate riding situations. The classics like the WTB Speed V or some of the new offerings from Fabric have enough padding to allow you to sit comfortably but not so much that interferes with you moving around. Mountain biking involves enough time out of the saddle that you don't have to get as picky as a you do for road biking or rails-to-trails riding. The unique consideration for mountain biking is the ability to move around on and off the saddle, you need to be able to shift your weight around while on the saddle and then you need to be able to get behind the saddle for extreme handling.
If you took the time to read this whole thing and are ready to take the time dialing in your ride. Feel free to call the shop, we're happy to answer any specific questions.