Suit jackets are defined often by the shoulders. The rest of the jacket hangs from the shoulders and the shoulders help define the face of the wearer. Also, nothing makes a suit look out of style quicker than the shoulders. If the shoulders are too big you will look like a football player and it will make you look shorter, since it will shorten the length of your neck.
If the shoulders are too wide than the suit will make you look like a Dick Tracy mobster and will also shorten you by adding un-needed width. I have had to throw out at least one suit, due to unfortunate shoulder pad width.
The best looking suits have natural shoulders. This means minimal to no padding in the shoulders, letting the suit fit to your natural shoulder shape. This will help you look taller and slimmer, since the shoulders will have a nice shape without the extra padding or width. Suits are supposed to help your silhouette, making you look more masculine and powerful. Natural shoulders will define the shoulders, defining your face without hindering your look without unnecessary bulk.
You can tell a suit has natural shoulders by feeling for the padding in the shoulder. How bulky is it? Also, make sure that the shoulders of the suit stop at your natural shoulder. Keep the shoulder natural and you will have a suit that flatters your body and makes people turn their heads.