Software that sells
When considering the question you might initially think about things such as usability, functionality, speed, stability, flexibility, the look and feel, the "user experience" (whatever that is), how easy it is to configure or personalise, it's ease of use or a range of other things. In a technical sense you could be correct because these are all product differentiators, but in reality you've missed the essence of the problem. These qualities are the things that make one product different from another, but they don't explain why one product sells better or worse than another. The truth is, that if you look at things this way then you're looking at the question from the wrong perspective. You're looking at it from a sellers perspective. You need to think like you're customers.
Marketers think like this all the time. It comes naturally to them and if you ask them what the difference is between products that sell and products that don't they'll tell you (if you find them at an honest moment) that it's not how well a product performs, how technically superior it may be over it's competitors or how flash something looks. What they'll tell you is that it's the products that ...
Give the customers what they want
Products that do this don't automatically sell well, but products that don't will never sell well, no matter how great they are marketed or promoted. If you want a case in point, look at Hollywood. The movie studios regularly make flops - massive, gigantic, worldwide phenomenal flops. Movies like Gigli, Torque, Battlefield Earth and more. Why do these movies flop? Well, it's not because of poor marketing.
It's because the paying public didn't get what they want. Most people (apart from the "artistic" set) watch movies for entertainment - they want to escape, to be able to live an adventure for the next 2 hours, to identify and care about the characters they are watching, to be awed at the special effects, to feel and emotional rush as the heroes get out of yet another jam, etc (your reasons may vary).
Why did Star Wars, Titanic, Indiana Jones, etc all become such massive hits? Because they gave the movie goer what they wanted and more - I can still watch Star Wars after seeing it umpteen times and enjoy myself as Luke blows the death star out of the sky. So do my kids. So does my wife. Having seen shockers like Torque where I felt completely overwhelmed at the absolute crap being shown on screen I can safely say that not getting that enjoyment gives you a string sense of feeling ripped off. Not really what I wanted.
So what is it that your customers want? Well that's a much harder question to answer. It's hard because I don't know who your customers are - but you do. When you talk to your customers, what is it that they say, what are they asking for, what attracts them to you in the first place? On the flip side (and this is harder to answer) if they are attracted to you and your products then what prevents them from purchasing your product? What is it about your product that sucks or what is it about your company that makes them nervous.
When I started with my current employer and first saw the product there were some things I noticed straight off the bat. The software we produce is business system and carries a great deal of functionality, but in places the interface was not really usable and in some places a bit kludgy. As a user I don't want the slickest interface imaginable, but if I'm going to being using a product and staring at a screen all day long I don't want it to be boxy & disorganised. I want something that looks and behaves in a contemporary manner and something that lets me do my job without getting in the way.
But that's my opinion, and I'm the seller so I could be incorrectly biased. What about the customer? What were they saying and how could I find out what they are thinking? It was actually pretty easy - I've got myself involved in presales activities, tender presentations, installations, etc. I listened to the people in the company who are facing the customers on a daily basis - the support people, the trainers, the implementation teams. They all talked about the visuals and the usability.
Based on this we put in a strong effort to improve the way the system behaved and looked. The end result was worth it - the responses from customers were very positive. The customers like the changes, they feel more comfortable using the system and as a result feel a lot better about the product.