It'll probably be a little while before I end up getting such a bike, but my current 80s road bike (Raleigh Technium 440), while OK for commuting and even some bike camping, isn't really the greatest fit for me and at some point I'll probably want to change it out for something I'm more comfortable on for long distances. Basically, I want to be able to keep an eye out for craigslist deals for bikes that would be good for this and want to know what features make a good long-distance ride.
The main requirements I'd have:
Plenty of gears for when I get tired/big hills
Able to carry a large (Carradice) saddlebag's worth of day-ride supplies, but doesn't need lots of weight capacity - I'm also looking at getting a dirt-road worthy rig for touring
Capacity for both fatter tires (32 or even 35 mm maybe) and fenders; I don't like riding rough roads on skinny tires
Keeps me in a comfortable position all day. I know fit is a major component of this but so is frame design, I just don't know what to look for.
Speed is less important than comfort, but still up there in case I'm riding with a group.
Edit: Also, examples of good frames to keep an eye out for would be great.