There are many, many things to consider with front kicks. They actually come in many flavors.
The most imporant things to consider are:
1) First and foremost, the force vector is tangential to the trajectory of the kick. And you get 100% energy transfer if you get that contact force vector orthogonal (perpendicular in all directions) to the surface you want to damage or move. This principle of physics explains why pointing the knee, leaving it still, and extending leg with quadriceps results in an "incorrect" kick. The force trajectory will be straight up, and actually parallel to the contact surface. So not only will you throw a perfectly useless kick (which may still have a nifty snap with your gi), but you will also put too much stress on your knee joint vs. all the other joints that can be involved with the kicking motion.
2) Do you want to thrust or snap? Thrusting involves "time on target." These are "stopping" and "crushing" techniques. Snapping involves more soft tissue damage. The biggest visual difference is how long the foot stays extended. A thrust kick is more like a (correct) Uechi sanchin thrust. (I've seen some folks try to snap that. Not correct in my book.) A snap kick is more like a Uechi back fist.
3) What surface of the foot do you want to kick with? The toe (sokusen), ball of foot, foot blade (sokuto), and heel are all perfectly correct versions of the same front kick (shomen geri). And a "Uechi side kick" is not a side kick. That's a misnomer. It's a front kick hitting with the foot blade (Soku*to means foot blade. Shu*to means hand blade. Soku*sen means foot somethingorother...probably related to the toe.)
4) Do you want to emphasize speed or power?
The "Rabesa style kick" mentioned above is what Art taught for years to folks who were in his sparring classes. It is optimized for speed. The foot follows a straight line from floor to target. There is a sufficient horizontal component of the force vector on contact to get the job done, and you get it done FAST. The knee still raises and lowers, but not much.
A kick optimized for power and "stopping" will involve more knee lift before extension. That will recruit the gluteal muscles more than the speed kick, and 100% of the force vector is horizontal - straight into the target.
5) "Primitive" kicks involve fewer joints, muscles, and degrees of freedom of motion. The best and most powerful kicks recruit most of the body, and the movements are quite complex. A perfect Uechi front snap kick uses:
* Extension of the ankle as kick goes foreward so that the kicking surface always faces the target - just like a cobra.
* Extension at the knee using the quadriceps.
* Movement of the knee using the glutes (hip extension).
* Rotation of the femurs (BOTH of them ) in the hip sockets. This prevents the "dipping" of the head and/or lifting of the heel of the support leg. It also can add energy to the motion. Ideally you can "cock the trigger" by overemphasizing the sanchin toe-turn-in. This IMO is why Uechika CAN BE masters of the kick off the front leg. It's the stored energy built in the sanchin stance!!
* Snapping of the pelvis (using lower abdominal muscles). Ideally you can "cock the trigger" a bit by letting the ass stick out just before the kick (allowing for some lumbar lordosis before firing the kick).
6) Remember that there is a valid "football kick". What is "wrong" according to rule number 1 above (for hitting a vertical surface of the body) is perfectly correct if you're trying to drive a guy's balls up into his ears. There the target is horizontal, so the trajectory at point of contact should involve a force vector that is perfectly vertical (orthogonal to the contact surface). This is a great self defense technique that we must never forget. Contact point is generally top of foot. Actually to me, I consider this a lot more like a roundhouse rather than a front kick. You just change the orientation 90 degrees.
Hope that helps.