For building and linking purposes, as well as manipulation effectiveness, I think the prim creation tool should also allow you to adjust the axis of a prim. Perhaps limiting it to a side, edge, or corner as well as the center would be nice, but being able to allow a user to adjust where the axis of a prim is located would greatly facilitate building as well as improve the look of composite prim objects.
This could also greatly help prim movement scripting, as well.
Potential impact: Potential reduction of prims used for building composite shapes, as well as scripting prim movement.