Oh they hurt. You have no padding. ow.
Bars: Your hands get ripped, If you happen to let go, you would be in BIG trouble! You could probably die (if you are doing a trick on it that is!) if you accidentally let go! Your arms could slip and you could get a long scrape on it, the same for your shin, ect..
Beam: You could very easily fall off it, if you do any sort of trick you could slid off, or crotch it (that kills!), you could hit your head (remember you can't have a helmet on), ect..
Floor: Back flips, you could land on your head, you could hurt your back if you landed wrong (actually you could hurt anything if you landed wrong), ect..
Vault: You could trip while you are running, when you jump on the spring board you could hit it wrong and hurt your ankle and hit the vault really hard, your hands could slip and you could go flying over it, you could hurt your back if you arch it to much, ect..
Just remember, you are only wearing a leotard. In any of the events in gymnastics you could sprain your ankle, elbow (I fell off bars twice and sprained both elbows), you could jam or dislocate anything, ect.. It is a really dangerous sport, next to football, boxing, wrestling, ect.. Why do I like it so much? I love the challenge!!