[jbeise]

Duplicate the original image (Ctrl+D) and right-click on the copy. Select Image -> Mode -> Grayscale. I don't know how this conversion works in GIMP, but I have read that Photoshop uses a standard mix of the RGB channels for their grayscale conversion: RED=30%, GREEN=59% and BLUE=11%. Supposedly this mix accounts for the eye's sensitivity to different colors. This formula does a pretty nice job in the general case, but some images do not work as well with it, particularly if the green channel component is not strong.

I suspect GIMP uses a similar formula. My experiments with the Channel Mixer support this.
if I use desaturate instead. Duplicate the original image (Ctrl+D) and right-click on the copy. Select Image -> Colors -> Desaturate. Unlike the grayscale mode change above, the channels are not remixed in different percentages, so we should expect different results.

A third method is to consider the red/green/blue channels of the image. Each one can be represented as an independent grayscale image. Right-click on the original image and select Image -> Mode -> Decompose. Select the RGB option and click OK.
Very often the green channel contains an excellent B&W version of the image. If nothing else, taking a look at the RGB decomposition is important to give you an idea of where the important information is in your image, and where the noise is.