As I do mostly illustrations, I'd like to share my ideas on this subject. When you think of an artwork or series, one of the issues you need to consider is the following:
Of course, what can be simplier than one-colour artwork? Silhouettes and illustrations in one line look stylish and they can be well bought for T-shirts, package or souvenir design because they are cheaper to print.
From simple to complex...
... is one step. Because some graphic artists create very difficult and detailed shapes (as the "Atacking Eagle in Flames 3" by Vallentin).
Next level is a picture which includes several plans. For example, the artwork "Cook" by Agau includes two plans: the girl in the front and the soft background. The "Profession Set: Photographer" by Ultrapop includes three plans: foreground with the grass, the main one - figure of photographer - and background with tree silhouettes and sky.
The other layer in illustration industry belongs to hand-made artworks which can be bought for a postcard or poster. As for me painting helps if you need to relax your soul, when you are tired to work on the computer. Also it helps to develop colour vision and feeling.
Those artists who found their own style are best-sellers. Why are still you not one of those?