Mirroring Bones Rotates Bones! I labeled every bone with the .L and turned on x axis mirror, nothing happened. I set the cursor do the origin, duplicated bones and scaled them by -1 across the mesh and the new bone rolls were different for no reason. I tried going to the armature mirror option and still the new bones rotated for no reason. Why can't blender manage to execute a basic mirror function?
Symmetry is a wonderful operator for borns editing, so mirror constraints are not necessary. It can be used as follows. 
In the edit mode, select L bone and press Armature > Symmetry in the header menu.