This is an excellent thread, and the timing is great -- "preparation season" when we're all looking at maps and asking "what if I....?...."
In trying not to duplicate the great lists above, I would add that having the right gear at the right moment can be lots of fun. If it's raining, having a poncho or jacket on hand. Or arm warmers on a cold morning. Or a neat little road morph pump to reduce the stress of a tire change. Or that lowest gear that wasn't "stock spec" on the bike when you bought it, and aren't you glad you have it with that next killer hill coming up.
It's also great to have CGOAB to archive the ride with words and photos and point people toward it. Even with my teeny tiny little mini tour last spring with my son, I've heard him mention with pride that "dad has a web site of the ride.... go to...... etc etc".
A special +1 to SRS' comment just above this post. Those first few pedal strokes of the day. <sigh>