I think there is a technical limit to things like sharpening, contrast and saturation. In all reality though, everything is up to the artist, but certain things are kind of a no-no from what I've learned in photography class. Using too much contrast will make your blacks go too inky and you'll lose details in the shadows, or too much sharpening will add halos in areas of intense local contrast or just make it slightly painful to look at, etc.
Also, certain genre's lend themselves to more processing then others. If your shooting for a newspaper, the most you'd want to do is crop, correct exposure/color and sharpen a little bit. Landscapes on the other hand or fine art lend themselves to more surrealistic colors and comprehensive exposure manipulation. But again, it's up to you. If your trying to use the images professionally, certain things just won't work.