A recent speed painting concept of mine:
As I have begun building up an illustration/concept art portfolio, I have wondered, what makes a good concept art piece? Concept art in and of itself is vastly different than illustration. As an artist is it sometimes good to have a solid grasp of both. From my research I have discovered the following:
Concept Art establishes mood, atmosphere, and scale. It should show value, space, and composition. Concept art should read well from a distance. Use value to establish a focal point and a clear idea. Perspective also is of great importance. Concept art is a means to an end. It isn't meant to be shown directly to an audience like illustration but handed to a team of 3D modelers and texture artists. Therefore if your perspective is off, it is harder for someone else to work with what you have. It may be confusing, or be impossible to replicate in a 3D environment.
Concept Art subject matter varies and so should your skill set. Concept art can be anything from character design, creature design, storyboards, machinery design, and environment design. Films usually have the budget to hire artists for each specific category of concept art but game designers usually need fewer artists with a wider range of skills. It's also a good idea to show both organic and inorganic elements.
A great resource for concept art is the site FZD School of Design. They have several video tutorials which go over everything I have just described above. Check them out if you have a chance.