Truth be told, I am a reformed black and white snob.
Before I went digital, I shot almost exclusively in black and white. Why the change? I learned to appreciate color as soon as I had to work to calibrate my printer for it. Suddenly, I paid more attention to color and I began to shoot with color in mind.
But let's say you are trying to convert color to black and white for the Monthly Special. Do some photos work better in black and white than others? Yes. Most definitely. But which ones? That's the hard part. Personal preference plays a huge role here; there are no set rules. Nevertheless, here are a few ideas to consider:
I'm envisioning something a teenager might like in their room rather than realism. This very contrasty and stylized (almost iPod ad-like) conversion emphasizes the silhouette and dramatizes the sky.
Possible rule #1: Extreme lighting converts well to black and white.
The above detail from a statue in the Louvre highlights the life-like body and beautiful folds of fabric created by the sculptor. I liked the color original, but seriously, how much color did it really have?
Possible rule #2: If the image is fairly monochromatic to begin with, it's a good candidate for black and white.
Technically, this isn't the best image, but I wanted to post it anyway because I like the old man stooped over the bin of books contrasted with the younger man going about his business and glancing sideways. Is he judging? Is he thinking about stopping to look at books as well even if it means arriving late to work? In color, the trees, awnings, and umbrellas in the background distract from the real subject matter.
Possible rule #3: If color distracts from the real focus try black and white.