Of course ergonomics are the most important factor for touring which is why I have a Bike Friday NWT. It was made to give me the riding position of the big wheeled touring bikes I used to ride yet it's physically smaller and folds. Also due to the 20" wheels there's more room to pack stuff above the wheels yet keep the center of gravity lower than with a traditional large wheeled touring bike. Other points worth mentioning:

• PowerGrips or the like (I prefer Origin8 Pro Grip straps) allow for efficient pedaling yet don't limit you to wearing one specific shoe.
• A good saddle is a must... Brooks work for me... I haven't worn bike shorts in 30 years and again, am not limited to just wearing bike specific shorts or pants yet I can rack up the miles in comfort.
• When it comes to tires thinner isn't necessarily better. Although I've found that there's a very slight improvement in speed when using thinner (28) Schwalbe Duranos the ride of wider (35) and lower pressure Schwalbe Marathon Plus tires offer a more comfortable ride day in and day out which means a lot to me on a bike with no suspension. They're also more puncture resistant.
• Gearing the bike with more options in smaller steps/increments within the speed range you spend most of your time in helps.