Hardtail mountain bikes, or mountain bikes with solid front forks, are excellent for adventure touring. They're tough, offer a comfortable riding position, handle well, are fun to ride and can carry gear easily. 29er wheels roll very well and will usually take MTB, cyclocross, hybrid and road tires. But most hardtail mountain bikes are sold for, well, mountain biking, and there's some things you can easily change to optimise the bike for touring on rough and variable terrain rather than mountain biking per se. Here's what I did to (both of) mine.
Both my hardtails have air front forks. These are designed mainly to soften the high-amplitude, low frequency shocks you get when riding trail - bumps, roots, rocks, small drops etc. The amount of pressure in the fork, and the release/rebound control if the fork has one, are dialled to the rider's expected weight, and if the bike has been set up for you by a LBS, the shop will set it for your weight using a chart of pressure vs body weight. Mine came out about 100psi. If you're not doing MTB-style trail riding all day, you can change the pressure in your fork so it's tuned more toward mitigating the high frequency, low amplitude shocks you're more likely to encounter riding steadily over rough surfaces all day; vibration damping, rather than shock absorbing. I run mine at about 150. This has the added advantage that when locked out, they're much closer to being actually locked out. I run mine locked out on all but the roughest terrain.
Even if the idea of this appals you, or your tech refuses to do it, you at least need to recalculate the pressure based on the weight of the gear you're carrying. Don't just add the weight of the front bag(s) directly over the fork - add the weight of all the gear. I'm usually about 9-12kg with everything.
The lockout on my Spesh is at the top of the right piston. So I never need to touch it. On the Giant, it's a remote lock on the bars, which is a pain as it's taking up space and denying me a completely flat central hand position. Getting rid of this is one of those little jobs...
Mountain bikers use a huge range of different tires and tire pressures depending on what type of riding they're doing, their weight, and the trail conditions. I normally run cyclocross tires on my hardtails, at 45-70 psi depending on what type of riding I'm doing. I don't need full-width MTB tires for what I do. With the pressures, basically the harder the better for fast rolling, softer tires will give you more grip but require more effort. If I am going out in the boonies, I'll run MTB tires at 35-45 psi. The fastest rolling MTB tires I've found so far are 2.1" Maxxis Crossmarks. They roll great on most surfaces, and don't actually give up much grip over softer tires - the only thing with them is that they're pretty noisy on the road.
I've been casually told by bike shops that my setup is "aggressive for a mountain bike". Maybe so, but it's better for what I do, and actually seems pretty close to what cross-country racers have. Every XC bike is different, but for me the rule of thumb is - get your backside at least as high as your hands. That should give you an efficient riding position without hurting your back and neck too much or pitching you over the front.
I guess one problem is that, with a large seat-post bag on, you can't use a dropper post to lower the seat when descending steep slopes, so it's a compromise about what feels most comfortable vs safest. I don't do long scary descents if I can avoid them, so I err more on the side of efficiency and riding comfort. My position on the XC is actually not that different to how I ride my adventure road bike, except the saddle is farther forward.
With the front roll bag on, and because the bars are fairly low, I can lean right over and rest my forearms on the nice soft bag while going into the wind. Or just have a wee sleep during boring sections.
One skill I find essential, absolutely essential, is setting the bike up so it's really well balanced and I can ride it, including pedalling short stretches, no hands. On long rides this makes a huge difference as I can sit up completely straight and stretch my back and neck. When climbing, and I do lots of climbing often at low speeds because of the weight of the packs, I use a position where I steer the bike with one hand right in the centre of the bars at the stem clamp, holding it as loosely as possible, push my hips forward and relax my back muscles, basically to get into the most low energy position possible and take all the load off my back muscles. It's also excellent for looking at the scenery and distracting from the pain in my legs.
One of my hardtails came with a flat bar, the other with a low riser. I like both - the setup is a factor of the stem length and angle, number / height of spacers, and how far forward your saddle is. On my Spesh which has the low risers, I angled them slightly so that the rise points backward toward me, after discovering that by doing this I got a lot more variation in position between holding the grips and holding the bars centrally, next to the stem clamp - basically the stem clamp is now slightly farther away than the grips. This especially helps going into the wind where I can get into quite a low position that's still quite comfortable.
Both bikes came with 700mm handlebars, to enable riders who have tentacles instead of arms to ride them. I trimmed them to 620 and 600 respectively, which means I can get in between trees.
I use flats for touring. Well, unless I'm riding over fist-sized chunks of France.