fiber orientation
Which way the fibers point. Point them all one way (aligned or unidirectional) and the part is superbly stiff along that line but floppy across it, like wood that splits easily along the grain but not across it. Scatter them randomly (a chopped-strand mat) and the part is more equal in every in-plane direction but weaker overall.
Along aligned continuous fibers (the longitudinal, isostrain case) stiffness follows the upper rule-of-mixtures bound and can be enormous; loaded across them (the transverse, isostress case) stiffness collapses toward the matrix value. A quick example: 60 percent carbon fiber (230 GPa) in epoxy (3 GPa) gives about 139 GPa along the fibers but only about 7 GPa across them, roughly a 20-times difference from direction alone.
This anisotropy is a design tool, not just a nuisance: you orient plies to match the loads. Laminates stack layers at chosen angles (e.g. 0/45/90 degrees) to get strength in several directions, and a pressure vessel winds fibers to resist hoop stress. Get the orientation wrong and a strong composite fails at a fraction of its rated load.
A fishing rod or a golf shaft is wound with fibers mostly along its length for bending stiffness, plus some angled layers to resist twisting.
Aligned buys peak performance in one direction; random buys uniformity at lower peak.
There is no free lunch: putting fibers in one direction robs the others. Random orientation buys uniformity at the price of peak performance; aligned buys peak performance at the price of directionality.