Prioritizing from survival to comfort:
Strong rims and fatter tires to help protecting rims. Decent hubs. Higher number of spokes. Steel or carbon fork. Whatever it takes to take the edge off the impact. The rider will feel the jolt no matter what, and in my experience my wrists tend to take it worse than my butt. It is always possible to post out of the saddle, softening the grip on the handlebars could be iffy at times. That's why I run my front tire about 10-15 psi lower than back one.
P.S. Thank you, cyccommute, for exploring the rider's part of equation. Frame should fit, preferably not weigh a ton, and that's just about it.
Last edited by sci_femme; 09-21-09 at 09:25 AM.