@Trav-man:
Go to the channels palette. Click on the red, green, and blue channels, looking at the difference each makes on the pic. There's your answer right there. With just about any pic, the channels all look very different. One section that may have no contrast in red may have a lot in blue or green.
When you desaturate, you're sucking the color out, but you have little control over the contrast. You almost always get better results when you use the channel mixer or do some selective work with the channels.
hope this helps.
