Any motorcycle can be used for touring but there are several necessary requirements that need to be met, and others considered to be desireable.
Over the years I've met countless motorcyclists on tour, on everything from a Honda Melody (moped type thing with a shopping basket on the front) to Gold Wings, Harleys and sports bikes. They all met the expectations and budgets of their owners which is probably the most important thing.
The Honda Melody was touring Scotland with it's septuagenarian owner, he was staying in Youth Hostels - and taking 2 months to do it in, I bet he had a brilliant time doing 30 or 40 miles a day and stopping off for a few days here and there. For eating up continental miles on the motorway a different class of machine altogether is required. Both of these activities can be classed as touring though.
A saddle with sufficient width and support is worth it's weight in gold, I once had an Africa Twin with a seat like a plank, I eventually got used to it, but it was a painful experience. My current bike has an aftermarket saddle (expensive) but it's "All day long", comfy. I did a 1000 mile day for an Iron Butt challenge on it with little discomfort.
The riding position needs to be comfortable, too much of a sports bike crouch and you will suffer from sore wrists, arms, shoulders and back - unless you are doing a motorway blast at high speed in which case you will probably be reasonably comfortable. For what I would call normal touring - on A and B roads at a more sedate pace a more upright riding position is needed.
Everyone is different and sometimes a set of bar risers can help. These are machined blocks designed for specific bikes that allow the handlebars to be mounted slightly higher than normal, an inch can make a big difference.
Reducing the wind pressure will stop you from getting tired so quickly, it will also help in reducing heat loss which can become a problem on a cooler day or in the spring and autumn. Even something as simple as an aftermarket handlebar fairing can make a noticeable difference. Many bikes are produced in 2 different styles, one with no fairing and one with a half fairing, go for the one with the half fairing if you're even thinking of touring (e.g. Suzuki Bandit). A full fairing is the way to go though for serious touring. Some big trail bikes offer as much weather protection as a touring bike due to the half fairing and the protection afforded by the fuel tank (e.g. BMW GS range).
Heated grips and/or handguards are useful for anything other than summer - and also for touring in Scandinavia or other places where the temperature is low. Some bikes (e.g. Honda Pan European and BMW RT1100/1150/1200 amongst others) have lower mounted rear view mirrors behind the bodywork that are designed to make a sheltered area just at the grips on the handlebars, they work very well.
I find the position of the footrests to be important for long days in the saddle. I don't like my knee to be at too much of an angle and some sports bikes (and sports tourers) have rests positioned so that the lower leg is folded well back giving a sharp angle at the knee. I find this causes reduced circulation after several hours or if wearing bulky clothes (thermals etc.). I try to get a bike that allows me to have my knee at something close to a right angle, or that allows me to move my legs about a bit. Some bikes with full fairings have little or no room to do this.
Don't forget about your pillion... most of above applies. Maybe moreso for an easy life and no complaints from behind!!
To propel you along at a respectable speed and to carry all your kit you will probably need a reasonably powerful machine. Over the years I've used such diverse bikes as a Honda Superdream (yeah, I know...), CX500, CB500, Africa Twin and various BMWs for big mileage touring. The bigger bikes make it so much easier and I would recommend a bike of at least 600cc for serious touring, some smaller capacity bikes are adequate, depending on the type of touring. For all day motorway work a smaller bike will be very tiring as it will need to be ridden quite hard, if the speeds are going to be less and the roads a bit more twisty then a smaller bike may be easier to handle. I've had some customers on machines less than 600cc which are fine for the rural roads of Ireland.
Sometimes I am asked "What is the ideal type of bike for touring on Irish roads"? I have to say that big trailbikes are my favoured weapon of choice... Long travel suspension to deal with the "little imperfections" of our rural road network, high seating position for a good view, wide bars for easy handling, tight turning circle for country lanes and town work, and usually controls are not cramped and fall easily to hand. A good, faired, touring bike would be 2nd choice.