Diabetic inserts are made with a material on top which closest durometer to human tissue.  The durometer is essentially the hardness of the material and with the materials on diabetic inserts, there is much less possibility for irritation, friction, calluses and blisters.  Regular inserts are what we typically refer to as functional orthotics and these are designed more for correcting painful deformities in the foot and can have a variety of different covers.