The (mis-)use of contrast in black and white photographs has been catching my eye of late.
Increasing overall contrast increases visual drama, drawing attention to and emphasizing the highlights and shadows of the scene. There's a sense of visual "pop" that comes with increased contrast. Increased contrast also visually connects an image to other photographs we've seen that use high contrast, or a more generally the full range of tones from blackest blacks to whitest whites. In the original conception of the full tonal range philosophy, of course, it was not necessary to have great masses of black and great masses of white, but by having those masses the photographer makes the connection to that philosophy unambiguous.
Also called acutance, this is usually achieved with some sort of "sharpen" control, of which there are several types. You can also get this sort of increased local contrast by fooling around with color filtering in the black and white conversion process, and probably half a dozen other ways. In all cases, the aim is to increase the apparent contrast between small areas that are close together, to increase the contrast at edges. This will usually steal from the overall contrast, by using up available tonal range on a small scale. Over-applied, it will create the unattractive halos of the tyro.
Heightened local contrast emphasizes detail and texture, and de-emphasizes larger forms and shapes. It creates a sense of busyness, and can create a sense of visual chaos. In addition, of course, you get an increased feeling of "sharpness" to the image, which can add a flavor of higher quality to the image. It feels like it was taken with better equipment, with sharper lenses (this is generally why novices love the "sharpen" control).
Both kinds of contrast control, when increased, stretch the tonal range out, to shove more of your image up into the whites and down into the blacks. Subtle changes in the midtones are spread apart and rendered less subtle. The midtones always suffer when applying contrast controls. Applying both a sharpening step, and then putting the overall contrast back in to compensate, can almost completely destroy the midtones. Your image appears rendered in blacks, whites, and not much else.
None of these effects is necessarily bad, but they can be very pronounced.
One needs, always, to consider the image and what you'd like the viewer to feel. If you want to create a sense of serenity and peace, for instance, perhaps applying some sharpening technique is the wrong thing to do -- does increased busyness, bordering on visual chaos, enhance anyone's sense of peace?
Calm grandeur? Perhaps a good does of overall contrast for drama, but very little sharpening.
Tension and unease? Does sharpening it heavily increase this? I think it's worth trying.
If you just want your image to look sharp, get a tripod and a good lens. Use the sharpen controls for mood, not sharpness.