Horse Power Loss ?

This issue or question came up on another post and some of us felt this should be a stand alone topic so here goes.
We hear of HP projections at the crank on a motor and then there are projections of rear wheel HP which is considerably less than that at the crank. What in your opinion or fact would be the contributing factors for this loss of HP at the rear wheels. We know there are many. Some losses can be eliminated or lessened cheaper than others. What are the remedies on a street car. Just to name a few that comes to my small mind:
Frame connectors........
torque boxes.........
sure grip rear........

What else