A big serve comes from creating a fully stretched position with your body and racket. This position is quite different from the "scratch your back with your racket" mantra that I grew up with.
Note three things about Roddick's great drop position here. First, notice the high elbow position. This comes from rotating your shoulders upwards (what I called the see-saw movement). If you drew a line from shoulder to shoulder here, it would be a slanting diagnal line. Second, notice the cocked wrist position. This wrist position leads to the third thing to notice -- Roddick's racket runs along the right side of his body (not straight down). This will add more distance for the racket and arm to pronate (which is the big generator of speed and power on this shot).
If you hover over the picture, you can see Roddick's awesome extension into the court. Watch how his arm and his racket are a straight line exteneded far forward into the court towrads the net. The big mistake players make is they try to "snap their wrist". This ends up short circuiting the pro like extension you see in this picture. Instead of "wrist snap" think "pronation and extension". Is there wrist movement here? Yes, of course there is. But the wrist movement simply aids the pronation and extension into the court. Many people incorrectly believe that the wrist itself is the source of power. It's not. It is just one small piece of a larger pronating and extending movement.