Design is very important in any product development. There are many aspects of design, considering the field that you are involved in. The design could include the structure of beam in a building or it could include the architecture of your application. But there is one design which is visible to the user - the interface design of the product. This aspect of design could make or break the product in the market place. It doesn't matter which segment of your market you are targetting, the interface design has to be efficient.
Mainly, I consider these three factors :
1. Functionality - What it can do?
2. Purpose - How important is the product?
3. Experience - How do you feel when you use it?
Its always few basic human needs that drive designer to make a product which suits user purpose. among them these days, user experience is in the top priority. I stumbled upon the slide below which throws some interesting light into the matter.