My five cents:
As mentioned there is a lot of things that influence how a load will behave, and it is often hard to predict in advance, but in general the more symmetrical a load is, the more stable it is.
A cylindrical bucket for concrete will not spin at all.
A bigbag will also usually not spin, but if it is loaded a bit bulky or a flap of the bag catches air, it could start to spin. This can also start suddenly after it has been flying stable for a while.
For square shaped loads, the lower the length to width ratio is, the more stable it will be.
The weight to drag ratio should also be as high as possible. A pallet weighing 800-1000 kg can fly quite nice, but a barack measuring 2,5x2,5x2,5m with the same weight is more prone to start spinning.
I guess it has to do with inertia. The larger the mass, the larger force you need to move it, and the airflow is more powerful if it can "grab" a larger area.
Rectangular loads like lumber and planks might not spin to bad, but as it slowly rotates, the vary in drag depending on if the short or long side is facing the airflow will make it vary between dragging behind and "falling" forward, and therefore dance around in all directions. You must therefore adjust the speed to where it is safe and comfortable enough. A more heavy and compact load of iron reinforcement bars will on the other hand fly smooth an you can fly as fast as the helicopter can.
How you strap up the load can also influence how it will fly. A pallet of bricks will probably fly quite nice and stable if you use two straps and choke them so the two lifting points are over the centerline of the load and at equal distance from the center. If you instead choose to choke them at opposite corners for better choking around the load it is more likely it will start to spin.
Some loads can be stabilized by horizontal airflow and fly nice, but start to spin once you reduce airspeed on the approach to delivery and vertical downwash start to influence it.
I hope this can help you a bit, but as other have said it is not possible to make a textbook about this, and it is more about having the experience so you can best predict what will happen.
Last edited by turbinturbin; 9th Nov 2014 at 13:36.