Quote of the moment:
A horse is a horse, of course, of course, unless you're in France. Then it gets... weird.

Home is where the heart is, perhaps, but home is definitely where you'd probably rather use the john.
Here's a list of the relative comfort factors of various places to take a dump "in the wild." I'm sure I missed some particularly gruesome ones, but I think it's a well-rounded list, all the same.