This is another topic in which opinions are commonly expressed. Ivan Viehoffs page is based on far more experience than these notes (by Colin), which may nonetheless be of some value.
We ride steel. Aluminium is lighter but less comfortable. Many people are suspicious of its durability and its tendency to fracture without warning.
We have no truck with it, though this may be mere conservatism. It adds weight and reduces reliability, and it gets in the way of panniers, but it may make rough ground easier to negotiate.
I now think that 26" is always better than 700C. Spare parts are easier to obtain, and the smaller wheels make it possible to provide a relaxed geometry for short riders. But you may prefer the look of 700C on a bike with drops.
Sun Rhynos are ridiculously heavy.
We used to put knobblies on our mountain bikes in wet periods, but can no longer be bothered. They dont confer much advantage. You may as well stick with smooth tyres. Knobblies are desperately hard work on tarmac.
I try to keep even fat tyres at high pressure (perhaps higher than the rated maximum), and for preference would never ride on less than 80 psi.
The BB height varies a lot between bikes, and is often given insufficient consideration. The Marin specifications show a height ranging from 10.5" for racing bikes, through 11.5" for hybrids, to 13.25" for technical mountain bikes. A high bottom bracket may make a bike excruciatingly painful for long rides (and may damage your knees).
Drops are best for aerodynamics. Straight bars make control easier on rough surfaces. Bar ends are brilliant for pulling down on on steep tarmac climbs drops cant compare with them.
Nothing less than a triple chainset should be considered. If you can ride on a double, youve gone to the wrong place.
Clipless pedals are the invention of the devil. Use toeclips if you dont wish to emulate the fate of hedgehogs.
The bell is the soul of the bike. It should be brightly coloured and childish.
Lightweight trekking shoes are good for cycling. Those with Goretex lining have stiffer soles. The drawback is the waterproofing. Whenever you get your feet wet water comes in at the ankles, and the Goretex prevents it from escaping. Im considering boring small holes in mine.
As I learn more about bikes my ideal mountain bike gets closer to a tourer, and my ideal tourer gets closer to a mountain bike. When mountain bikes first came in they werent far from tourers. Nowadays theyre getting more and more extreme, while touring bikes are copying some of their good features. So the debate between advocates of tourers and of MTBs is rather futile. The best bike for your purpose may be either.
In fact my mountain bike is a Marin Muirwoods, an inexpensive bike sold for commuting which (with substituted wheels) has carried me over the Himalayas and the Andes. It was an excellent purchase.