What I find with JPG files is that pushing adjustments too far tends to cause a certain 'blocky' coarseness and loss of tonal gradation, particularly in areas of fairly even tone. With RAW I can normally push the adjustments that much further before this starts to happen.

It is possible to achieve excellent results in JPG as long as you take care not to over adjust and get the image as right 'in camera' as possible. Basically RAW allows you more leeway to recover images that are poorly exposed or more importantly images where the exposure range of the scene exceeds that of the camera - as long as you expose to keep the highlights from blowing out RAW is more effective for recovering deep shadow detail with acceptable quality than JPEG.