This. You can more or less get any kind of stiffness, with any material by changing the geometry. I guess the different feel that people have when using frames

This. You can more or less get any kind of stiffness, with any material by changing the geometry. I guess the different feel that people have when using frames of various materials is more due to geometric constraints of the frame. A frame with the same geometry (which includes tube thickness etc) and a different material will behave differently. But in theory you could construct a frame with equal stiffness in all directions from other materials. However, you might run into other problems, like fatigue resistance or tensile strength or sth with a particular geometry. That will then force you to choose another geometry for other materials (e.g. aluminum frames will have a bigger tube diameter than steel frames, due to buckling of the tubes - if you want equal weight)