Choosing a sleeping bag normally starts with the decision of synthetic fill vs. a natural down fill. At This Tribe, the Outdoor Shop, we focus on the needs of the military customer who wants a lightweight sleeping bag for use in the field, and the warm-when-wet characteristic of a synthetic fill is normally the best choice. The Snugpak Softies all use a synthetic fill. The Softie 3 supplements this with a heat reflecting layer of Reflectatherm. Down bags perform better when dry.
Some high-end down sleeping bags use a waterproof breathable outer to keep the bag as dry as possible.
Sleeping bag shape is also a key design factor, and most serious outdoor sleeping bags are 'mummy' shaped. This reduces the excess internal space around the legs so that there is less air to heat up / keep warm.
In addition to the Sleeping Bag fill and shape, the internal structure of the bag is key. The fill must be held in place. The simplest method, normally seen on bottom-end camping bags is "stitch-through" where lines of stitching criss-cross the bag and go right through the outer and the fill. The obvious disadvantage is the development of cold spots around the stitching. More serious bags including the Snugpak Softie range will use one of a variety of techniques to overcome this, such as overlapping internal 'baffles'.
Finally sleeping bags have all sorts of extra features that are of more or less use. For example important in Snugpak Softie bags is a reinforced foot area inside the bag, to deal with boots. Most bags will have some form of zip flap to help insulate the thermal weak spot that is the zip, and there may be internal stowage pockets.