From my experience it is having the ability to create beautiful images through any medium and translate it on to products. There are may avenues for design, fabric, home goods, apparel, stationery, packaging and more. Each field takes a special type of design, size and colour for it to work on that item. Knowledge of drawing, layout, printing, working with soft ware like Photoshop and Illustrator are all necessary.
Some products are large and therefore need repeating patterns or images that can be enlarged with out loosing quality. Therefore the surface designer has to not only have an artistic flair but also has to have the knowledge to not only translate it on to an item but also know other information like trends, style, what will sell, what is the market, how can it be profitable how to sell it and in comes business administration as well. So the job of a surface designer is demanding takes a lot of learning, talent, observation and a variety of complex skills to be successful.
I have touched only the tip of the iceberg on this subject, there is much more to know about a surface designer and most of all one has to love it because it is a lot of work, but I do love it.