I agree with BOF, and want to elaborate a bit further. I believe that pitching mechanics cannot be disected into different aspects of a motion because every body part leads to another. If one aspect of the motion is done improperly, then chances are the other aspects of the motion will be incorrect as well.
When I am teaching a lesson or when I notice a mechanical flaw on myself while throwing, I like to take things all the way back to the basics. I am a HUGE fan of "short boxes" as we call them. Short boxes are just flat ground bullpens, 45-50 feet away, at maybe 60% velocity. During these, the sole focus is mechanical work, repeating pitch consistency and honing in on issues that need to be dealt with. I throw short boxes every day I am not going to be pitching live off a mound, including days in which I long toss. I believe that these are crucial for mechanical development and overall success.
When discussing the front side glove action, I believe that you can't neglect the other aspects of throwing a pitch either. As I've stated, the way to generate the most velocity is to use all of the energy in your body and generate 100% of that towards home plate. Any excess motion in any other direction would be detrimental to the throwing process. For the sake of this thread, let's fast forward the mechanics to the balance point.
The above two pictures are at relatively the same time in the pitching motion. At this point, the body begins to transfer all of the energy. Note: It is a fallacy that the weight transfer occurs from the back leg to the front leg. The weight should always be generally in the middle of the body. I always tell kids "your weight should be in a straight line from the ground, up through your you-know-what, to your belly button, to the top of your head". The differential in weight transfer occurs by taking the middle of your torso...your hips...and beginning to move your entire body forward. The same concept applies to hitting. Some say "transfer the weight from the back foot to the front foot". But wouldn't hitters consider this lunging? I'm pretty sure Albert Pujols never hits a ball off his front knee. And I'm pretty sure Roy Halladay never throws a ball off his front knee.
The above picture is when the body exits the balance point position and begins to descend towards home plate. This is where BOF's mention of the "open door" swinging of the leg comes in. If the front foot flies out towards 3B (for righties) or 1B (for lefties), that is causing excess longititudinal movement. From the balance point, take your foot straight back down towards the ground, and then out towards home plate.
Another thing I'd like to point out is that the foot should not lead the body towards the plate. The hips are the strongest part of a pitcher's body and the most important aspect of generating torque, which results in velocity. So, therefore, the hips lead the motion. My pitching coach always says "throw your butt at the plate." He doesn't mean this in a literally sense, but the concept is to help keep your hips closed off and leading towards your destination.
Once your hips lead your body, then the glove arm becomes a factor. If you've accomplished the lower body aspect of the pitching motion, then the rest, in my opinion, is easier. The hips are what drives the torso towards home plate, and the glove arm is what helps the throwing arm follow in suit. The concept of pointing your glove at your target is a bit much, in my opinion, and rather old school. I have always been taught to keep your thumb down, and point your elbow at the target. Once your front foot is planted and you have followed suit with the lead elbow/thumb down concept, you should be just about ready to release the ball.
Note: Many instructors believe that an "inverted W" is the proper way to set your mechanics at this point. However, I believe that the immense torque created on the throwing elbow in this short amount of time causes many to be destined to injury (Ie: Mark Prior, Stephen Strasburg). In my opinion, your arms should form a shape that almost looks like half of a Nazi symbol, with the throwing arm at a 90 degree angle towards the sky, shoulders perpendicular, and the glove arm at a 90 degree angle towards the ground (with the thumb down still, of course).
The above picture is the point in the motion after the "half Nazi symbol" has been formed, and I am now reaching into the actual act of throwing the ball. Notice that the thumb is starting to rotate, like BOF said.
The above photo is just before release. Notice that the glove, which was once thumb-down facing the ground, is now driven into the chest. The torque in which you can open up your hips, combined with the torque that you can use your elbow to drive your lead arm back into this area, will generate velocity. The counter-acting motion of the hips moving forward and the front arm moving backwards is what many people refer to as "getting over the front side." When I teach lessons, I use this analogy: Pretend a person is riding a bike, and they come to a 3 foot high wall. They don't see the wall, and crash into it. What happens? Well, the wall stops their movement, causing the bike to stop. But they continue to move and will go flying over the wall. Relating this to pitching in a metaphorical sense, the wall is your front leg, the bike is your hips and the person falling off the bike is your upper body.