+1 to what Coelcanth said. Let's assume you are going to be doing a typical tour with long hours spent in the saddle and baggage attached to the bike. (maybe you're looking to do more of a "credit card" tour, but even so you'd still have the long hours) The main component is a comfortable frame that fits you and is as light as possible while still being strong enough to take the extra weight of the bags, and it ideally should have the rack mounting braze-on bits for your racks and fenders. Next most important items are: 1. a strong wheelset with good (wide) touring tires, 2. wide-range gearing, i.e. a triple crankset, 3. extra-good brakes like cantilevers, since you will have more weight with that loaded touring rig and it needs to stop. Do a search in this forum for "touring bike" as well as checking out everything that Sheldon might have on the topic...I'd also sniff around on this archive:
http://search.bikelist.org/?SearchString=touring
a quick search on the word "touring" came up with 19,000 hits...you'll want to cull a little.