A lot of people tend to choose a sofa just on looks, but, although aesthetics are important, a sofa is first and foremost meant to be used and therefore must be comfortable.
The comfort of a sofa can be subjective depending on how a person sits, but most of us appreciate a sofa we can sink into rather than one where we have to perch on top. Where to put your feet can be an issue, as some of us like to lie back and stretch out. If this is the same with you, then your sofa needs to be long enough to accommodate this or have low arms so that your feet can dangle over the edge. If the arms are too high they can also make it difficult to rest your head comfortably.
When reclining for some time, watching TV for example, you need somewhere to put your feet. You need enough seat width to be able to stretch out, or a footstool at matching, or slightly lower height.
The height of the armrests can be important, since, if too high or vertical, they can be uncomfortable to lean your head against. Low armrests are good if you want to lie lengthways and dangle your feet over them.
Deeper sofas tend to be more comfortable, and, if soft enough can take on the shape of your body, giving support where needed. Soft cushions of feather or down are the comfiest but do need plumping up from time to time. Foam is adequate but has a bouncy feeling, and is more like a car seat than a luxurious sofa.
A soft comfortable sofa is an investment that should last a long time. It is important to take time to really try out a sofa properly before buying. Certainly you should not just buy a sofa online because it looks good, as it may be difficult to return it if you find it's not very comfortable.
People are different shapes and sizes so there may not be one sofa style that suits all. You may want to consider the needs of those who will use it most. You may want to consider matching footstools if the sofa is to be used for long periods of time.
Another possibility is to consider a sofa-bed if your space is very limited, but remember that these tend to sacrifice some comfort for the sake of versatility, so much depends on your priorities.
Find out the type of filling used in the sofa. Top of the range sofas tend to use natural materials like goose down, feathers, hair or wool, whilst cheaper sofas may rely on a combination of some natural wool and foam, or be completely filled with foam or material scraps.
If the cushions will unzip you can check the filling of them, which, ideally should be firm and hold together, not made of millions of small fibres or bits of material, or the cushions won't hold their shape.
Where fabrics are joined with seams you check that these are secure and not likely to open up. Last of all, check the cushions by unzipping and looking at the padding inside. It should feel firm so that the cushions don't sag.