Warning: Do not read this write-up and go off, thinking you are magically able to throw ninja stars through concrete and kick faster than sound. Someone will come along and crush you good.
As with many aspects of this almost xenomorphically beautiful art, Wing Tsun's stance is very misunderstood. By stance, I here refer to placing the legs in the IRAS (internally rotated adduction stance) and holding the hands in man sao and wu sao. This is taught as the position that one begins a fight from, as it closely resembles a relatively natural posture, making for very little transition time. In fact, a Wing Tsun stylist, anticipating combat, can stand to make this transition time even smaller, essentially infinitesimal.
The correct posture of the legs is found by placing the feet together, then rotating them to point outwards, then pivoting from the balls of the feet to acheive a "pigeon-toed" leg position with the feet placed roughly under the shoulders. The knees should point inwards. Despite the initial oddity of this stance, it is important to remember that it is relatively natural compared to most kung fu stances, in which one tends to squat so low that toilet humor begins to seem the appropriate response.
More important than the correct angle of the knees or the correct distance of the feet is the correct pelvic and spinal alignment. Many Wing Tsun/Wing Chun stylists stand with a curve to the spine that makes them look like a question mark with fists; this is so, so wrong, and can't be maintained against the realistic force of an opponent. The stance, done correctly, is quite difficult (read: essentially impossible) to uproot when done correctly. (This is not to imply that Wing Tsun advocates force-on-force tactics. Quite the opposite. However, to not simply be swept away, one needs to apply adequate force. Thus, "yielding" is a choice, not simply a consequence of looking like the hunchback of Notre Dame with a dizzy spell.
Standing in IRAS, one should rotate the pelvis forward. This should create tension in the legs, as it creates an opposing pressure to the internal rotation of the knees. This pressure is used to quickly launch the Wing Tsun stylist forward into an offensive jin bo ("arrow-step").
The upper body should feel as if it were stacked upon the hips. This is hard to explain, but I can say only that it is a sensation one experiences from correct alignment. It means a lack of muscular exertion, but it also means that the upper-body is placed well over the hips and the spine is straight, almost as if being elongated. The head should be held back and up-right. You might have a tendency to hunch your head down, or begin to stare at the flurry of hands before you. However, you will be rewarded with a punch to the head, quite possibly on accident. Great grand master Yip Man said that he would pretend his head was made of glass. Good advice. Despite what movies like Braveheart would suggest, it is a huge tactical error to place your general on the front lines.
Finally, the hands. As stated before, they are placed in the man sao and wu sao positions ("searching arm" and "protecting arm," respectively). It doesn't matter which hand is forward, as the Wing Tsun stance is symmetrical.
A conclusion: Wing Tsun might as well be the lost fourth internal art. The attention to detail, internal understanding, and reactive sensitivity in Wing Tsun is matched only by particularly good and well-taught internal styles. However, as far as quality of techniques and training methods go, we would need another WU.