I would say the most common injuries by far are related to muscle strains, tendons and ligaments etc like you could see in any sport. Cuts to the head are fairly common, as are the fabulous 'cauliflower ears' caused from bleeding inside your ear. But this is from being squashed and abraded, not hit. I haven't watched too much American football, but it seems a lot more stop-start, with considerably more changing of players. In Rugby you are running around for 80 minutes, so more athletic injuries rather than collision related. Perhaps having to pace yourself also lessens the hits? Spinal injuries seem very rare, and are much more likely due to some kind of contortion rather than impact.

Concussion does happen, but normally seems to be an accidental clash of heads or falling on a knee, perhaps. When it happened to me it was a result of me running to join a ruck (think lots of people pushing each other over a ball on the the floor after a player has been tackled), which kind of split in half and someones head slipped through to catch me in the face. In the top level, if the referee sees you suffer a head injury or get knocked out you are sent off to see the doctor for 10 minutes. If he suspects concussion you wont play again for two weeks. A Welsh international (George North, Wales is Rugby mad btw) got a concussion on his return from a concussion break and this resulted in him spending the best part of a year out of the game!

I don't think head on head hits are common at all. When you tackle in Rugby you have to attempt to wrap your arms around the player, and tackles are much more 'tacklers shoulder to midriff or legs'. I think being winded or hurting your stomach muscles is most likely. you certainly cannot tackle above shoulder high. Re: protective equipment, I wore non at all. Gum shields are common, as is electrical tape around your ears and some petroleum jelly on your eye-brows! Now a number of players where a 'scrum-cap' which is a close fitting lightly padded cloth device to help stop cuts and abrasion to your head. Professional players often wear a kind of shoulder pad designed to protect your collar bone, but it barely makes a dent in your shirt it is so small!