Six jours

I agree that thickness has a smaller effect on stiffness than does diameter, but wall thickness does have a very significant effect. I have built frames that are nearly useless for my size and weight simply because the wall thickness was inadequate. The exact same size and geometry made with the exact same steel of the exact same diameter, but with a wall thickness .2 mm greater, becomes an absolutely wonderful frame for me.

And you are correct re. shock absorption and tubing thickness, but that is not what I was alluding to. Many folks, myself included, enjoy the feel of a frame with a certain amount of lateral flex. It has nothing to do with road shock and everything to do with how a bike feels to my legs; how it responds to pedaling input. If the frame is too stiff, it feels heavy and "dead" to me. Other people, of course, aren't bothered by that at all, but to me, it makes the difference between a remarkable frame and one that is just "doing a job".