All these suggestions about exploding equipment and balls made of sharp things are fine and dandy, but I think it's more interesting to try to come up with a more subtle change that VASTLY increases the danger of a sport.
Take football (soccer), for example. When a member of a team gets a foul, the opposing team gets a penalty kick. No, I don't mean they get a free shot at the goal. I mean one of the players on the opposing team gets to kick the offending player as hard as they can. Seeing how strong some of the players are, with a little bit of training, it wouldn't be hard to injure a player badly enough that they could be out for a season, or even their entire career. I imagine fatalities would also be fairly common.
What I think this thread illustrates, more than how badly our parents failed to raise us into decent human beings, is that having reasonable safeguards against bodily injury in sports is important because it makes people actually want to play sports. I don't know about you, but I'm not too keen on playing a sport where the ball is filled with nitroglycerin and will blow me up if I don't catch it like a lobbed egg. Or where a really fit guy in cleats kicks me in the nuts so hard that one of them shoots up into my torso and punctures my lung.
