Loft
Loft is the number of cubic inches one ounce of down will fill under specific conditions: The larger, more mature the bird, the higher the loft of its down.

A loft test is done in a Plexiglas cylinder of a standard size, under exacting conditions of specific temperature and humidity, with a sample of precisely one ounce of down. There is a disk of specific weight that sits on top of the sample to simulate the load that the covering fabric exerts in a finished item.

The highest loft ever to be consistently tested is 1000 but this is extremely rare.

Very high qualities will test at 800.

This illustration shows the loft of different grades of down. Each tube is the same size and filled with 1 ounce of down. It is easy to see that a higher quality, more mature down fills more space for the same weight. This explains why a high quality down product will be much lighter and at the same time much puffier than a lower quality item. It takes much less of a high lofting down to fill any product

             

Density

This describes how dense the fibers of down are at its center. The denser the down, the better it traps air, and the better insulator it is. High density is found only in very mature downs.

The photos show the difference between a down plumule with high density (Right) and a down plumule with low density (Left).

 
Low density down
 
High density down

Cling

Eiderdown is the only down where cling is specific to its species. Other downs must be very mature to develop this trait. Tiny hooks develop as the down matures and these hooks cause down clusters to cling together giving a more even layer of insulation. Since it is costly to maintain birds to this level of maturity, the resulting down is very expensive, but exceptional.

 
A very mature White Goose Down   Eiderdown is the only down where cling is specific to its species

Breathability
Down has a remarkable ability to wick away moisture. It allows water vapour to pass right through it, keeping the sleeper comfortable, with none of the clamminess that is often felt with synthetics.

 

Cleanliness
High quality downs are carefully washed, rinsed and dried, using machinery developed for this specific purpose. Special sanitizing processes are used to ensure the down is hypoallergenic.
When this process is complete the finished product has a very high level of cleanliness.

To view a video detailing howdown is readied for use, watch this docmentary detailing howa duvet is made.