Black & White
The decision to turn an image from color to black and white rests largely on the question, "What divides your image?" Since most of us humans perceive color (i.e. we aren't color blind), we tend to separate scenes by color. Black and white images are separated by luminosity, that is, the contrast between light and dark portions of a scene ... Example time:
Big Sur in B&W
The ocean is separated from the coast by the contrast of the blue water and green land. While the clouds are a little more accentuated in the black and white, the primary subject is lost because the ocean and land have similiar brightness levels.
Yosemite in B&W
Here, I would place the color image squarely in the OK category. There's some decent separation between the blue sky, yellow rock wall, darker area surrounding the waterfall, and the waterfall itself, but the true subject is less stunning because of the distracting color. By turning the image black and white, the waterfall jumps out at the viewer and everything else serves as a nice background to the long exposure.