Arms - If space is tight, choose a sofa with slender arms to help maximize your seating space
Legs - Space beneath your sofa can create the impression of more room space so consider a sofa on legs.
Low back vs high back - A low back sofa can be ideal if you want it to sit under a window. If you are looking for back and neck support a high back sofa would be more comfortable and offer a more supportive sit. Consider the seat height and depth, check it is comfortable and feels right for you. Consider the seat height and depth, check it is comfortable and feels right.
Other options are ventional cushion back or scatter back or if space is restricted a more compact sofa maybe more suitable.
Colour - statement or subtle? If you want your sofa to be the focal point of your living room, a bold, eye-catching colour can really bring a room to life. If you want your sofa to blend into your room, subtle neutral tones suit most colour schemes and will allow you to accent and have an easy revamp with new scatter cushions or a throw.