I agree with many of the comments above as to what they notice, though my interpretation differs.
First, considering Roddick: His oddity is that he would essentially launch his serve with the racket intentionally no where near the usual top of the racquet lift/swing back. That has several effects I would not try to copy. He discovered the technique fooling around in practice as a teenager...and liked it. That technique pretty much requires that the initiation of the swing differ for the second serve, which is a losing concept. I'll never forget watching Fed outserve him in the extended Wimbledon final.
Your tossing arm. I agree with all that start with that: The tossing arm has an important role. I agree with all who say you should get the arm all the way up, relatively vertical.
The tossing arm plays a positioning role, a timing role, and a rotational velocity-boosting role. Correct use gets your tossing shoulder high and keeps it high until you WANT the tossing arm/shoulder to come down....which is when your racquet's going back, your legs are extending, and you want your UB rotation, your hitting shoulder rise. The timing role is big: You really want to synchronize your toss/drop of the tossing arm to your SIDE... to your leg extension and your racquet's passage over the top and back. Watch some pros in slo mo.
You really don't want your tossing arm to drop until your leg extension starts. Tossing arm elbow pull-in: If you try it a few times without a ball, you'll find that if you let the arm drop a little (with muscle...) to gain some momentum, then pulling the elbow in will accelerate your UB around just a bit. You want that to happen as your racquet hits its lowest point, to help power the hitting elbow up. (It's the same, really, as timing your forehand off-arm elbow pull-in to the instant of your initial forward UB rotation, forward motion of the hitting hand, in order to add a well-timed if minor boost to acceleration of the UB/shoulder rotation.
Your shoulders come around (not over) too far, in my view. Consider looking at, say, Fed and Sampras slo mo clips, and form your own opinion. Two very different serves, but neither comes around so fully (not talking about coming over with the hitting shoulder, but around.)
I would guess the tossing arm synchronization issue and the slight over-rotation issue affect your control up into contact ESPECIALLY on second twist or slice serves. (If you're going to copy Roddick, you'd better do it perfectly in every detail.....)
Fix the tossing arm use, especially sychronization, come more over and less around with your hitting shoulder, and you've got a kick a_s serve.
