Good modelling - Personally I think that this could do with some volumetric lighting effects and perhaps a hint of motion blur on the spheres to create the atmosphere (like you did on Anchor and Dead Tree). Also watch the camera angle - The angle used in this render would work well as part of a fly-through animation however it falls a little short as a still shot. A lot of 3D artists make the same mistake with camera angles and push the shot out of the viewers visual context. (Remember Judges have the same visual context). Tip: get a book on Industrial Photography and study the camera angles used in the real world.

Thanks for the constructive criticism, tattie. Since I got my digital camera ('Photorealistic Leaf' image...), I've been thinking more and more about real-world camera angles, so I'll definitely keep that in mind. The spheres (planets) in the Pillar Building render weren't actually meant to be 'moving', but now that I think about it, the composition would have benefited greatly from a motion blur (Of course, I wouldn't have had time to do the render with blur to meet the challenge deadline ;) Thanks again for the comments.