[black & white modification from the original image by Andrew Gillis]
I've been attempting, perhaps unwisely, to reverse-engineer the judging of Nikon's famous Small World competition. An ideal assay of the judges' criteria would be to compare traits of the 115 images selected for official recognition against traits of the thousands of non-winning submissions. Lacking access to all the entries, though, I couldn't do that. Instead, I looked at commonalities among the publicly available judges' selections.
What made the winners?
After perusing the galleries a few times, I think it is this: simplicity.
Most images show a starkly demarcated subject against a smooth, single-tone background. The lack of extraneous, non-subject elements grants an elemental clarity to the images.
To draw out what I mean by clarity, I converted images to a black-and-white bitmap, as you see in this post. With color and shading dropped from the equation, winning images still unequivocably project their subject.
[modified from an original by Marta Guervos]
[modified from an original by Stephen S. Nagy]
Color improves each of these selections, of course, but I do not think it a coincidence that they all start on a foundation of strong silhouettes. If you are planning on submitting to the 2012 contest, such simplicity is worth bearing in mind.