I would like to see more power coming from your lower body. I would like to see more effort coming from your upper body rotation. You are simply tapping the ball in my opinion and are not fully gathering and using your efforts and energy into the ball.
Let's talk about your toss and the movement of your head. Although you toss in front of you, on many of the serves your head moves back under the ball (at least from the camera angle I am viewing). This means you are not always hitting the ball with it in front of you. This can contribute to an arm that has to slow down as you may sometimes lose your eye contact with the ball. This also can contribute to an inconsistent serve. Some are great when you don't move so much under the ball and some are not so great when you do move under the ball.
The non-dominant arm is especially important to really help relax the shoulder region of the hitting arm - you have to trust the relaxation. The entire hitting arm/shoulder needs to be as loose as a noodle and be ready to easily and effortlessly release and come forward as the racquet is accelerating into the ball. The moment you tighten up, the fast your arm will slow down before contact. Bringing the non-dominant arm into the chest area will help accelerate and allow the shoulder to let loose and throw the racquet into the ball.
Check out Roddick: http://www.youtube.com/watch?v=hnfAx...related&search
Check out Federer's head (the best you can) when he makes contact.
Also, the more you can incorporate your hips, legs, and feet into your serve the more you will be able to use your shoulder rotation, arm extension, and energy into the ball. Someday, you will be able to do this.
Check out Safins serve, lots of hip stretch, legs, and toe push-off. It is a very active and powerful serve from the ground up. take a close look at his arm/shoulder region. It really is propelled by his shoulder rotation and has a lot of relaxation in it. http://www.youtube.com/watch?v=zT4q3...elated&search=