Developing Power.

There is a lot to be said for being able to throw a hard punch. There is also a lot to be said for being able to throw a fast punch. If you can through one you might not be able to throw the other however. For me a powerful punch is one that has a large mass released at the end, simple enough. For me however a fast punch, is a punch that has the shortest time span from conception to impact, not the speed of the arm while in transit.
The elements necessary for a powerful punch are a good stance, one that helps you drive your technique, and good body structure on top of your stance to also aid the travel of your punch to and into the designated target. There is one other element needed to hit someone and that is timing, but that is a whole different story.
The elements for a fast punch are relaxation, relaxation, and intent. There is nothing stopping someone throwing a fast powerful punch but the timing of each of the elements has to be just so, before you even think about the timing necessary to land the punch.
Fast and powerful punches both come with a set of dangers which we train to avoid. These are injuries based around the impact.
Bone alignment.
If you land a powerful punch without your wrist in line with the angle of the attack, you will end up with a bent strained wrist and less able to retaliate than your opponent, as the punch will loose power as your wrist absorbs the impact.
Round vs Straight punches, alignment for round and straight attacks differ. Round punches such as ridge hand or hook punch rely on the arm delivering the blow to be supported by muscle. This requires all involved driving muscles to be at a less than 90degrees on poi (point of impact) this is where muscles have their power range, 90-20 degrees. Straight attacks rely on the power that is being transfered by the arm to be supported with the body, so basically elbows in and everything associated with the hips from the head to the toes moving the weight of the attacker forward into the punch.
In power punches, this happens early and slowly, in speed punches this happens late and fast. The faster that the body comes in line the faster the punch lands the more impact it will have.

This might all seem obvious but I want a reference to talk about fast and slow attacks in the future.

...edit...
I do know how to spell through and also throw, I don't know when to go to bed...