I am intrigued by the concept of the simple cross. As beings in a world that is defined by three dimensions, we are intimately familiar, right down to the cellular level, by horizontality and verticality. I think this is why a cross is one of our most universal and fundamental symbols or marks, of an importance only equaled by the circle.
I like making crosses in my stitching. (This form is known as an upright cross stitch.) The piece I just finished gave me the opportunity to think a lot about why crosses feel so natural to make.
Here is a bit about the meaning of the simple cross (+) in other cultures. In ancient Greece, it represented the four elements of earth, air, fire, and water. In the ancient Middle East it represented the four cardinal directions as well as the four winds. In Buddhist tradition it represents crossed thunderbolts, which show the power of Buddha's wisdom. In Aztec mythology it represents a meeting place or sacred crossroads. In Japanese sashiko, it represents me, which means "eye", carrying protective powers. (And I like the pun on "I".) The symbol can be a medicine wheel or a mark denoting place or a universal sign of healing. And of course it is a connection between heaven and earth.
It is also very interesting as a stitch. Since a regular X stitch crosses the warp and weft threads at an angle it tends to sit more on top of the cloth, while the + meshes more evenly with the fabric. It requires a bit more effort to sew, but I'm not sure if that is just from being more used to regular cross stitch or if the motion of the hand is just more naturally suited to a diagonal motion.