http://www.gtgtandems.com/tech/materials.html

Weight is just about how optimal your parts are worth for the energy you use to carry them
Stiffness is the ability for the parts to transfer the energy
Aerodynamics is (obviously) the load against the friction of the wind.

If you don't get the part about the flex, if you've ever ridden a mountain bike with suspension forks when you get off the saddle to sprint, the front forks absorb almost all the weight and power you put in to it making your effort useless.

I read somewhere (I'll sift through my bookmarks to find it) that moving parts weight affects performance more than static parts.