Speed - Speed is very important. But quickness and suddenness are even more important. Don't build up in speed. If you do, you will tend to miss against a person with movement, even though your punches are fast at full extension. This is because there is a discernible buildup in your acceleration.
Relaxation is important for speed.
Don't tighten your fist up until you're almost fully extended. It describes a failure to shift the weight off of one foot and onto the other when throwing a power punch. Barrel of a gun - Look down your punching arm like you're looking down the barrel of a gun. This will help that arm to provide cover for your chin on that side while you're punching. Common mistake is for people to leave their chin open on the side of the arm they are punching with.