Probably pretty OT, but striking is quite interesting. If you strike slowly, you will move your opponent. Imagine a glacier. It has really incredible momentum because it is super massive. However it is moving only a few centimeters a year. If you stand next to it, it will eventually push you, but will do no damage at all. You just slide. Of course if you are rooted to the spot, it will grind you up. Glaciers can dig lakes and destroy mountains.
Now imagine a car (I'm imagining a Herbie style VW Beetle) with the same momentum. It will be going at almost the speed of light. If it hits you, you will vaporise whether you are connected to the ground or not.
Generally speaking, if you hit slowly you will do less damage than if you hit quickly. In many martial arts, you are taught not to use your strength because it is hard to recruit only non-opposing muscle fibres. Instead you want to relax and strike very, very quickly (quick hint: each joint can multiply the speed -- strike like a whip, not a hammer). There are many reasons involving physics why faster strikes do more damage, but I can't really explain in such a small space (and I'm not really qualified anyway).
When you get hit by a very fast strike when you are wearing rigid armour, the impact wave moves through you. The faster the strike, the more it will affect you. Even if it is damped by padding, it can easily knock the breath out of you. A really good kick can break bones. Slower kicks can be used to position the person, but they will do almost no damage.
Punches are less effective for a couple of reasons. First, the levers are smaller, so the final speed is usually slower (there are some tricks you can use, though). Next, the contact area is smaller. This is fantastic against unarmoured opponents because you concentrate the impact. For armoured opponents, it is already nullified. The rebound will injure your hand instead. Kicks with the heal and ball of the feet have enough protection to deal with it. I'd personally never do it with the top of my foot or hand, but there are some people who train enough to protect those areas that you could do it. Finally, people always assume that the initial hit does the damage, but remember the glacier. If the person is rooted to the ground, then the strike will actually rebound. You must align your bones so that rebound is directed into the earth. This gives you a huge effective mass. Imagine some one throwing a stick at you and hitting you end on. Now imagine running (at the same speed) into a stick (end on) that is poking out of a wall. The first one will hurt. The second one will take your head off.
It's easier to do this with certain kicks than punches (because your torso is a big lever and you are punching from the top of it). There are martial arts that train effective stances for punches, so it really depends on the style -- I am very opinionated about this subject, so will refrain from commenting more than saying that effective stances for punches are probably different than most people imagine.
Anyway, I apologise to the people not interested in RW martial arts and return you to your regularly scheduled fantasy martial arts :-)