When you are buying furniture always check to see what is on the inside under the fabric. Upholstery furniture should begin with a very solid base. It should be ONE piece. There is a company right now marketing furniture that comes in parts. If the arm gets soiled, just order up a new arm and replace it. If the middle cushion section is not holding up, order the middle section. I can not warn enough about buying furniture this way. If you are replacing parts yourself, isn’t there a possibility that those pieces will fall apart all by themselves?
The springs should be sturdy and at minimum 8-way hand tied. What that means basically is that the springs are tied to other springs in 8 different directions. It provides support. Really good furniture will either have extra springs or more ties in the back and sides to prevent cushions from scooting down.
The padding sits upon the springs and has many different fillings. They sometimes loose their fluffiness and can either be replaced or fluffed.