Probably most photographers will agree that controlling the exposure is one of the most important and hardest parts in making a quality image. However, modern cameras with through-the-lens metering and matrix metering and several auto-exposure modes in addition make getting perfect exposures as easy as it never has been before. However, sometimes specific lighting conditions can fool the meter of the camera, or the operator can make a mistake which will lead to a poor exposure. In the past, all images that had poor exposure condition were most often thrown away since there was no way to fix them. Nowadays, many pictures with poor exposure can be fixed and lead to an acceptable condition with help of digital darkroom.
In the past, great attention was paid not only to shooting the original picture but to processing the film as well. Traditionally the negative was put in an enlarger during processing in a darkroom and a print of it was then produced. The exposure problems could be then corrected by burning and dodging different area of an image. This was extremely important while working in a black and white darkroom. A lot of materials about exposure, developing, and printing black and white images were written by the great nature photographer Ansel Adams, who developed the Zone System to calculate exposure and development periods. Ansel often said, "The negative is the score and the print is the performance".
Digital cameras have much wider exposure latitude than the film ones have, so if you're using a digital camera you have many more possibilities in regulating your exposure, especially if you're shooting pictures in RAW file format. More latitude is achieved with RAW files since you can adjust each one of them individually and control the exposure up to 2 stops in either direction.
In color photography, you can change development times to have some control over exposure, but black and white film sure has much more latitude than color film, there's no question in that. With negatives, you can fix some other exposure problems in the enlarger; but again the latitude with color is not as great as with black and white. You have even less control if you're shooting slides or transparencies. So to get a good exposure right away in-camera is extremely important since the film can and often does become your final statement. There is even a smaller tolerance in latitude when final print is translated from a slide.
Modern photographers have much more control over exposure and contrast then their colleagues in the past could even imagine due to the digital darkroom. The ultimate picture can be adjusted in numerous ways and many problems can be fixed before printing; however, you still need to be very careful when making a picture. There is an old saying "Garbage In - Garbage Out", which works perfectly in the digital photography world. If you can't get details in a shadow or highlight in the camera, there will be none. Nevertheless, those problems can also be fixed to some point...