After the frame, the next major area of difference between an inexpensive sofa and a more expensive sofa is the seating suspension.
There are 3 main types of seating suspension for sofas.
First is a web suspension.
This type is where the furniture manufacturer runs webbing from front to back and sometimes side to side and attaches it to the sofa frame. This gives a little bit of suspension for the seat cushions to sit on and is usually found in lower quality sofas because it is quick, easy and inexpensive to build.
One caveat to this generalization is that certain very low, sleek styles need to use this type of suspension to achieve the modern look no matter how high the quality of the sofa is. I have found a few high end manufacturers that use web seating but the webbing materials that they use are extremely high quality, expensive and will last. A high quality web seating will always have the webbing run very close together and run front to back AND side to side creating a trampoline-like seating deck.
No-sag wire is the next most common suspension system used in building a sofa.
No-sag wire is an S shaped wire that runs from the front rail of the sofa to the back rail of the sofa and arches up slightly in the centre giving a bit of spring to the seating system on the sofa. Typically this is used in better quality sofas and will provide a comfortable base for you to sit on for quite a few years.
Again with no-sag construction there are varying degrees of quality within this suspension system and like the web seating the closer the S-shaped wires are placed together the better.